Lithium secondary batteries and nonaqueous electrolyte for use in the same

Abstract

A lithium secondary battery comprising: a positive electrode and a negative electrode which each has a specific composition and specific properties; and a nonaqueous electrolyte which contains a cyclic siloxane compound represented by general formula (1), fluorosilane compound represented by general formula (2), compound represented by general formula (3), compound having an S—F bond in the molecule, nitric acid salt, nitrous acid salt, monofluorophosphoric acid salt, difluorophosphoric acid salt, acetic acid salt, or propionic acid salt in an amount of 10 ppm or more of the whole nonaqueous electrolyte. This lithium secondary battery has a high capacity, long life, and high output.[In general formula (1), R1 and R2 are an organic group having 1-12 carbon atoms and n is an integer of 3-10. In general formula (2), R3 to R5 are an organic group having 1-12 carbon atoms; x is an integer of 1-3; and p, q, and r each are an integer of 0-3, provided that 1≤p+q+r≤3. In general formula (3), R6 to R8 are an organic group having 1-12 carbon atoms and symbol A is a group constituted of H, C, N, O, F, S, Si, and / or P.]

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser. No. 17 / 017,862 filed Sep. 11, 2020 which is a continuation of U.S. patent application Ser. No. 16 / 394,100 filed Apr. 25, 2019, which is a continuation of U.S. patent application Ser. No. 15 / 291,314, filed Oct. 12, 2016, which is a continuation of U.S. patent application Ser. No. 14 / 660,517, filed Mar. 17, 2015, which is a continuation of U.S. application Ser. No. 13 / 614,215, filed Sep. 13, 2012, which is a continuation application of U.S. patent application Ser. No. 12 / 090,655, filed Jun. 5, 2009, which is the U.S. National Stage of International Patent Application No. PCT / JP06 / 320882, filed Oct. 19, 2006, the disclosures of which are incorporated herein by reference in their entireties. This application claims priority to each of the following Japanese patent application, listed with their filing dates, the disclosures of which are incorporated herein by reference in their entireties.

[0002] 2005-305368JapanOct. 20, 20052005-305300JapanOct. 20, 20052005-314043JapanOct. 28, 20052005-314260JapanOct. 28, 20052005-331255JapanNov. 16, 20052005-331362JapanNov. 16, 20052005-331391JapanNov. 16, 20052005-331477JapanNov. 16, 20052005-331513JapanNov. 16, 20052005-331585JapanNov. 16, 20052005-332173JapanNov. 16, 20052005-339794JapanNov. 25, 20052005-344732JapanNov. 29, 20052005-343629JapanNov. 29, 20052005-349052JapanDec. 2, 20052005-353005JapanDec. 7, 20052005-359061JapanDec. 13, 20052005-367747JapanDec. 21, 20052005-369824JapanDec. 22, 20052005-370024JapanDec. 22, 20052005-377366JapanDec. 28, 20052006-005622JapanJan. 13, 20062006-013664JapanJan. 23, 20062006-019863JapanJan. 27, 20062006-019879JapanJan. 27, 2006TECHNICAL FIELD

[0003] The present invention relates to lithium secondary batteries and a nonaqueous electrolyte for use therein. More particularly, the invention relates to: lithium secondary batteries which comprise a nonaqueous electrolyte for lithium secondary batteries which contains a specific ingredient and a positive electrode and a negative electrode each having a specific composition and specific properties and capable of occluding / releasing lithium, and which are excellent especially in low-temperature discharge characteristics and have a high capacity, long life, and high output; and the nonaqueous electrolyte for use in these batteries.BACKGROUND ART

[0004] With the recent trend toward size reduction in electronic appliances, secondary batteries are increasingly required to have a higher capacity. Attention is hence focused on lithium secondary batteries, which have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries.

[0005] Lithium secondary batteries are used in various applications because they are high-capacity secondary batteries. In these applications, however, the lithium secondary batteries are mainly used as batteries of relatively small sizes as in, e.g., portable telephones. In the future, the batteries are expected to be used in a wider range of applications as large batteries for motor vehicles, etc. Although output is especially required of large batteries, mere enlargement of conventional small batteries results in insufficient performances. Various improvements in battery materials for an output improvement have been proposed (see, for example, patent documents 1 to 25 and non-patent document 1). However, a sufficient output has not been obtained so far, and a further improvement is desired.

[0006] Patent Document 1: JP-A-2005-071749

[0007] Patent Document 2: JP-A-2005-123180

[0008] Patent Document 3: JP-A-2001-206722

[0009] Patent Document 4: JP-A-2003-267732

[0010] Patent Document 5: JP-A-2001-015108

[0011] Patent Document 6: WO 2003 / 34518

[0012] Patent Document 7: JP-A-11-067270

[0013] Patent Document 8: JP-A-61-168512

[0014] Patent Document 9: JP-A-6-275263

[0015] Patent Document 10: JP-A-2000-340232

[0016] Patent Document 11: JP-A-2005-235397

[0017] Patent Document 12: JP-A-11-031509

[0018] Patent Document 13: JP-A-3-055770

[0019] Patent Document 14: JP-A-2004-071458

[0020] Patent Document 15: JP-A-2004-087459

[0021] Patent Document 16: JP-A-10-270074

[0022] Patent Document 17: JP-A-2002-075440

[0023] Patent Document 18: JP-A-10-270075

[0024] Patent Document 19: JP-A-8-045545

[0025] Patent Document 20: JP-A-2001-006729

[0026] Patent Document 21: JP-A-10-050342

[0027] Patent Document 22: JP-A-9-106835

[0028] Patent Document 23: JP-A-2000-058116

[0029] Patent Document 24: JP-A-2001-015158

[0030] Patent Document 25: JP-A-2005-306619

[0031] Non-Patent Document 1: J. Electrochem. Soc., 145, L1 (1998)DISCLOSURE OF THE INVENTIONProblems that the Invention is to Solve

[0032] The invention has been achieved in view of such background-art techniques. An object of the invention is to provide lithium secondary batteries which, even when fabricated so as to have a larger size, have a high capacity, long life, and high output.Means for Solving the Problems

[0033] The present inventors made intensive investigations in view of the problems described above. As a result, they have found that a lithium secondary battery having a high capacity, long life, and high output is obtained by using a positive electrode and a negative electrode each having a specific composition and specific properties and further using a nonaqueous electrolyte containing a compound selected from a specific group. The invention has been thus completed.

[0034] The invention provides a lithium secondary battery at least comprising: an electrode group comprising a positive electrode, a negative electrode, and a microporous separator interposed between the electrodes; and a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, the positive electrode and the negative electrode each comprising a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing a lithium ion, wherein the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by the following general formula (1):

[0035] [wherein R1 and R2 may be the same or different and represent an organic group which has 1-12 carbon atoms and n represents an integer of 3-10],fluorosilane compounds represented by the following general formula (2):[Ka-2]SiFxR3pR4gR5r  (2)[wherein R3 to R5 may be the same or different and represent an organic group which has 1-12 carbon atoms; x represents an integer of 1-3; and p, q, and r each represents an integer of 0-3, provided that 1≤p+q+r≤3], compounds represented by the following general formula (3):

[0036] [wherein R6 to R8 may be the same or different and represent an organic group which has 1-12 carbon atoms and symbol A represents a group constituted of H, C, N, O, F, S, Si, and / or P],compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, difluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of 10 ppm or more of the whole nonaqueous electrolyte, andthe positive electrode is any positive electrode selected from the group consisting of the following positive electrode [1] to positive electrode [5]:

[0037] positive electrode [1]: a positive electrode containing a positive-electrode active material containing manganese;

[0038] positive electrode [2]: a positive electrode containing a positive-electrode active material having a composition represented by the following composition formula (4):LixNi(1−y−z)CoyMzO2  composition formula (4)[wherein M represents at least one element selected from the group consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; y represents a number satisfying 0.05≤y≤0.5; and z represents a number satisfying 0.01≤z≤0.5];

[0039] positive electrode [3]: a positive electrode containing a positive-electrode active material selected from the group consisting of the following (a) to (d):

[0040] (a) a positive-electrode active material having a BET specific surface area of from 0.4 m2 / g to 2 m2 / g,

[0041] (b) a positive-electrode active material having an average primary-particle diameter of from 0.1 μm to 2 μm,

[0042] (c) a positive-electrode active material having a median diameter d50 of from 1 μm to 20 μm,

[0043] (d) a positive-electrode active material having a tap density of from 1.3 g / cm3 to 2.7 g / cm3;

[0044] positive electrode [4]: a positive electrode satisfying any requirement selected from the group consisting of the following (e) to (f):

[0045] (e) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material, a conductive material, and a binder on a current collector, wherein the content of the conductive material in the positive-electrode active-material layer is in the range of from 6% by mass to 20% by mass,

[0046] (f) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the positive-electrode active-material layer has a density in the range of from 1.7 g / cm3 to 3.5 g / cm3,

[0047] (g) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the ratio of the thickness of the positive-electrode active-material layer to the thickness of the current collector, i.e., the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector), is in the range of from 1.6 to 20; and

[0048] positive electrode [5]: a positive electrode containing two or more positive-electrode active materials differing in composition.

[0049] The invention further provides a lithium secondary battery at least comprising: an electrode group comprising a positive electrode, a negative electrode, and a microporous separator interposed between the electrodes; and a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, and the positive electrode and the negative electrode each comprising a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing a lithium ion,

[0050] wherein the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1) given above, fluorosilane compounds represented by general formula (2) given above, compounds represented by general formula (3) given above, compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, difluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of 10 ppm or more of the whole nonaqueous electrolyte, andthe negative electrode is any negative electrode selected from the group consisting of the following negative electrode [1] to negative electrode

[10] :

[0051] negative electrode [1]: a negative electrode containing as a negative-electrode active material two or more carbonaceous substances differing in crystallinity;

[0052] negative electrode [2]: a negative electrode containing as a negative-electrode active material an amorphous carbonaceous substance which, when examined by wide-angle X-ray diffractometry, has an interplanar spacing (d002) for the (002) planes of 0.337 nm or larger and a crystallite size (Lc) of 80 nm or smaller and which, in an examination by argon ion laser Raman spectroscopy, has a Raman R value of 0.2 or higher defined as the ratio of the peak intensity at 1,360 cm−1 to the peak intensity at 1,580 cm−1;

[0053] negative electrode [3]: a negative electrode containing as a negative-electrode active material a titanium-containing metal oxide capable of occluding and releasing lithium;

[0054] negative electrode [4]: a negative electrode containing as a negative-electrode active material a carbonaceous substance having a roundness of 0.85 or higher and a surface functional-group amount O / C value of from 0 to 0.01;

[0055] negative electrode [5]: a negative electrode containing as a negative-electrode active material an orientation-differing-carbon composite comprising two or more carbonaceous substances differing in orientation;

[0056] negative electrode [6]: a negative electrode containing, as a negative-electrode active material, graphitic carbon particles which have a roundness of 0.85 or higher and an interplanar spacing (d002) for the (002) planes of smaller than 0.337 nm as determined by wide-angle X-ray diffractometry and which, in an examination by argon ion laser Raman spectroscopy, have a Raman R value of from 0.12 to 0.8 defined as the ratio of the peak intensity at 1,360 cm−1 to the peak intensity at 1,580 cm−1;

[0057] negative electrode [7]: a negative electrode containing as a negative-electrode active material a multielement-containing negative-electrode active material (C) which contains at least one of a lithium-occluding metal (A) selected from the group consisting of Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, and Sb and / or a lithium-occluding alloy (B) and further contains C and / or N as element Z;

[0058] negative electrode [8]: a negative electrode containing as a negative-electrode active material two or more negative-electrode active materials differing in property;

[0059] negative electrode [9]: a negative electrode containing a negative-electrode active material which has a tap density of 0.1 g / cm3 or higher and has a pore volume, in terms of the volume of particle pores corresponding to the diameter range of from 0.01 μm to 1 μm as determined by mercury porosimetry, of 0.01 mL / g or larger; and

[0060] negative electrode

[10] : a negative electrode which, when charged to 60% of a nominal capacity, has a reaction resistance in a negative-electrode opposing cell of 500 Ω or lower.

[0061] The invention furthermore provides a lithium secondary battery at least comprising: an electrode group comprising a positive electrode, a negative electrode, and a microporous separator interposed between the electrodes; and a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, and the positive electrode and the negative electrode each comprising a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing a lithium ion,

[0062] wherein the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1) given above, fluorosilane compounds represented by general formula (2) given above, compounds represented by general formula (3) given above, compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, difluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of 10 ppm or more of the whole nonaqueous electrolyte, andthe nonaqueous electrolyte satisfies any requirement selected from the group consisting of the following electrolyte [1] to electrolyte [9]:

[0063] electrolyte [1]: the nonaqueous solvent as a component of the electrolyte is a mixed solvent at least containing ethylene carbonate and the proportion of the ethylene carbonate to the whole nonaqueous solvent is from 1% by volume to 25% by volume;

[0064] electrolyte [2]: the nonaqueous solvent as a component of the electrolyte contains at least one asymmetric chain carbonate and the proportion of the asymmetric chain carbonate to the whole nonaqueous solvent is from 5% by volume to 90% by volume;

[0065] electrolyte [3]: the nonaqueous solvent as a component of the electrolyte contains at least one chain carboxylic acid ester;

[0066] electrolyte [4]: the nonaqueous solvent as a component of the electrolyte contains at least one solvent having a flash point of 70° C. or higher in an amount of 60% by volume or larger based on the whole nonaqueous solvent;

[0067] electrolyte [5]: the lithium salt as a component of the electrolyte comprise LiN (CnF2n+1SO2)2 (wherein n is an integer of 1-4) and / or lithium bis(oxalato)borate;

[0068] electrolyte [6]: the lithium salt as a component of the electrolyte comprise a fluorine-containinglithiumsalt and the nonaqueous electrolyte contains hydrogen fluoride (HF) in an amount of from 10 ppm to 300 ppm of the whole electrolyte;

[0069] electrolyte [7]: the electrolyte contains vinylene carbonate and the content of the vinylene carbonate is in the range of from 0.001% by mass to 3% by mass based on the whole electrolyte;

[0070] electrolyte [8]: the electrolyte further contains at least one compound selected from the group consisting of compounds represented by general formula (4), heterocyclic compounds containing nitrogen and / or sulfur, cyclic carboxylic acid esters, and fluorine-containing cyclic carbonates in an amount of from 0.001% by mass to 5% by mass based on the whole nonaqueous electrolyte:

[0071] [wherein R9 to R12 may be the same or different and each represent a group constituted of one or more elements selected from the group consisting of H, C, N, O, F, S, and P]; and electrolyte [9]: the electrolyte further contains an overcharge inhibitor.

[0072] The secondary batteries described above each especially preferably are one which has any property selected from the group consisting of the following (1) to (3):(1) the overall electrode area of the positive electrode is at least 20 times the surface area of the case of the secondary battery;(2) the secondary battery has a direct-current resistance component of 20 milliohms (mΩ) or less; and(3) the battery elements held in each battery case of the secondary battery have an electric capacity of 3 ampere-hours (Ah) or higher.

[0073] The invention still further provides a lithium secondary battery at least comprising: an electrode group comprising a positive electrode, a negative electrode, and a microporous separator interposed between the electrodes; and a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, and the positive electrode and the negative electrode each comprising a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing a lithium ion,

[0074] wherein the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1) given above, fluorosilane compounds represented by general formula (2) given above, compounds represented by general formula (3) given above, compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, difluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of 10 ppm or more of the whole nonaqueous electrolyte, wherein the secondary battery has any property selected from the group consisting of the following structure [1] to structure [6]:

[0075] structure [1]: the overall electrode area of the positive electrode is at least 20 times the surface area of the case of the secondary battery;

[0076] structure [2]: the secondary battery has a direct-current resistance component of 20 milliohms (mΩ) or less;

[0077] structure [3]: the battery elements held in each battery case of the secondary battery have an electric capacity of 3 ampere-hours (Ah) or higher;

[0078] structure [4]: the current collectors of the positive electrode and negative electrode of the secondary battery each are made of a metallic material, and the metallic material of each current collector has been bonded to a conductor for outwardly discharging current by any of spot welding, high-frequency welding, and ultrasonic welding;

[0079] structure [5]: the case of the secondary battery is made of aluminum or an aluminum alloy; and structure [6]: the battery case of the secondary battery is made of a casing material in which at least part of the battery inner side comprises a sheet formed from a thermoplastic resin and with which the electrode group can be enclosed by placing the electrode group in the casing material and heat sealing layers of the thermoplastic resin to each other.

[0080] The invention still further provides a nonaqueous electrolyte for secondary battery which at least comprises a nonaqueous solvent and a lithium salt, wherein

[0081] the nonaqueous electrolyte contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1) given above, fluorosilane compounds represented by general formula (2) given above, compounds represented by general formula (3) given above, compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, difluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of 10 ppm or more of the whole nonaqueous electrolyte, and

[0082] the nonaqueous electrolyte satisfies any requirement selected from the group consisting of the following electrolyte

[0083] to electrolyte [9]:

[0084] electrolyte [1]: the nonaqueous solvent as a component of the electrolyte is a mixed solvent at least containing ethylene carbonate and the proportion of the ethylene carbonate to the whole nonaqueous solvent is from 1% by volume to 25% by volume;

[0085] electrolyte [2]: the nonaqueous solvent as a component of the electrolyte contains at least one asymmetric chain carbonate and the proportion of the asymmetric chain carbonate to the whole nonaqueous solvent is from 5% by volume to 90% by volume;

[0086] electrolyte [3]: the nonaqueous solvent as a component of the electrolyte contains at least one chain carboxylic acid ester;

[0087] electrolyte [4]: the nonaqueous solvent as a component of the electrolyte contains at least one solvent having a flash point of 70° C. or higher in an amount of 60% by volume or larger based on the whole nonaqueous solvent;

[0088] electrolyte [5]: the lithium salt as a component of the electrolyte comprise LiN (CnF2n+1SO2)2 (wherein n is an integer of 1-4) and / or lithium bis(oxalato)borate;

[0089] electrolyte [6]: the lithium salt as a component of the electrolyte comprise a fluorine-containinglithiumsalt and the nonaqueous electrolyte contains hydrogen fluoride (HF) in an amount of from 10 ppm to 300 ppm of the whole electrolyte;

[0090] electrolyte [7]: the electrolyte contains vinylene carbonate and the content of the vinylene carbonate is in the range of from 0.001% by mass to 3% by mass based on the whole electrolyte;

[0091] electrolyte [8]: the electrolyte further contains at least one compound selected from the group consisting of compounds represented by general formula (4) given above, heterocyclic compounds containing nitrogen and / or sulfur, cyclic carboxylic acid esters, and fluorine-containing cyclic carbonates in an amount of from 0.001% by mass to 5% by mass based on the whole nonaqueous electrolyte; and electrolyte [9]: the electrolyte further contains an overcharge inhibitor.Advantages of the Invention

[0092] According to the invention, lithium secondary batteries especially having excellent low-temperature discharge characteristics can be provided. The embodiments of the invention which will be described later produce, for example, the following effects.Positive Electrode [1]:

[0093] According to the invention, a lithium secondary battery especially suitable for use as a large battery in, e.g., applications such as motor vehicles can be provided because a long life and a high output are obtained with less expensive materials.Positive Electrode [2]:

[0094] According to the invention, a lithium secondary battery having a high capacity, long life, and high output can be provided, and a lithium secondary battery especially suitable for use as a large battery in, e.g., applications such as motor vehicles can be provided.Positive Electrode [3]:

[0095] According to the invention, a lithium secondary battery having a long life and a high output is obtained and, hence, a lithium secondary battery especially suitable for use as a large battery in, e.g., applications such as motor vehicles can be provided.Positive Electrode [4]:

[0096] According to the invention, a lithium secondary battery having a high output is obtained and, hence, a lithium secondary battery especially suitable for use as a large battery in, e.g., applications such as motor vehicles can be provided.Positive Electrode [5]:

[0097] According to the invention, a lithium secondary battery which has a high capacity, long life, and high output and is inhibited from decreasing in battery capacity or output with repetitions of charge / discharge (has excellent suitability for repetitions of charge / discharge (cycle characteristics)) is obtained. Consequently, a lithium secondary battery especially suitable for use as a large battery in, e.g., applications such as motor vehicles can be provided.Negative Electrode [1]:

[0098] According to the invention, a lithium secondary battery can be provided which can realize high-output characteristics from an initial to a final stage in a cycle while retaining improved suitability for cycling and which, even after having deteriorated through charge / discharge cycling, retains high-output characteristics. This battery is suitable especially for use as a large battery.Negative Electrode [2]:

[0099] According to the invention, a lithium secondary battery having satisfactory suitability for short-time high-current-density charge / discharge can be provided.Negative Electrode [3]:

[0100] According to the invention, a lithium secondary battery having a low output resistance and enabling the effective utilization of energy can be provided.Negative Electrode [4]:

[0101] According to the invention, a lithium secondary battery improved in high-temperature storability at a low state of charge can be provided.Negative Electrode [5]:

[0102] According to the invention, a lithium secondary battery can be provided which can retain satisfactory suitability for long-term repetitions of charge / discharge at a low state of charge.Negative Electrode [6]:

[0103] According to the invention, a lithium secondary battery showing a quick output recovery from a low-output state at low temperatures can be provided.Negative Electrode [7]:

[0104] According to the invention, a lithium secondary battery can be provided which, even when fabricated so as to have a larger size, has a high capacity and is satisfactorily chargeable.Negative Electrode [8]:

[0105] According to the invention, a lithium secondary battery having both satisfactory cycle characteristics and a satisfactory low-temperature output can be provided.Negative Electrodes [9] and

[10] :

[0106] According to the invention, a lithium secondary battery can be provided which, even when fabricated so as to have a larger size, has a high capacity retention through cycling and can attain a satisfactory battery life, and which can attain a high output even after a charge / discharge cycle test. This battery has both of these properties.Electrolytic Solution [1]:

[0107] According to the invention, the low-temperature characteristics of a nonaqueous-electrolyte secondary battery can be greatly improved. More specifically, the low-temperature characteristics can be improved without impairing cycle characteristics and storability.Electrolytic Solution [2]:

[0108] According to the invention, a nonaqueous electrolyte for secondary battery greatly improved in both cycle characteristics and low-temperature characteristics and a secondary battery which employs this nonaqueous electrolyte and is excellent in these performances can be provided.Electrolytic Solution [3]:

[0109] According to the invention, a nonaqueous electrolyte for secondary battery greatly improved in low-temperature output characteristics and a secondary battery having such performances can be provided.Electrolytic Solution [4]:

[0110] According to the invention, a nonaqueous electrolyte for secondary battery can be provided which has an electrolyte composition less apt to pose problems associated with the incorporation of a large amount of a low-viscosity solvent, such as the problem that salt precipitation, a decrease in flashpoint, and an increase in internal pressure at high temperatures are apt to occur due to solvent vaporization, and which, despite this, enables high low-temperature characteristics and high output characteristics to be retained. A secondary battery having such performances can also be provided.Electrolytic Solution [5]:

[0111] According to the invention, a nonaqueous electrolyte for secondary battery which greatly improves output characteristics and brings about excellent high-temperature storability and excellent cycle characteristics and a secondary battery having such performances can be provided.Electrolytic Solution [6]:

[0112] According to the invention, a nonaqueous electrolyte for secondary battery excellent in high-temperature storability and cycle characteristics and greatly improved in output characteristics can be provided.Electrolytic Solution [7]:

[0113] According to the invention, a secondary battery greatly improved in both cycle characteristics and low-temperature characteristics can be provided.Electrolytic Solution [8]:

[0114] According to the invention, a nonaqueous electrolyte for secondary battery greatly improved in low-temperature discharge characteristics and excellent in high-temperature storability and cycle characteristics and a secondary battery having such performances can be provide.Electrolytic Solution [9]:

[0115] According to the invention, a secondary battery satisfying both of high rate characteristics and safety in overcharge can be provided.Structures [1] to [5]:

[0116] According to the invention, a lithium secondary battery which has a high capacity, long life, and high output and is highly safe in overcharge is obtained. Consequently, a lithium secondary battery especially suitable for use as a large battery in applications such as, e.g., motor vehicles can be provided.Structure [6]:

[0117] According to the invention, a lithium secondary battery which has a high capacity, long life, and high output, is reduced in gas evolution, and is highly safe in overcharge is obtained. Consequently, a lithium secondary battery especially suitable for use as a large battery in applications such as, e.g., motor vehicles can be provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0118] FIG. 1(A) shows a SEM of the thin film negative electrode produced in Negative Electrode Production 1.

[0119] FIG. 1(B) shows the Si distribution of thin film negative electrode produced in Negative Electrode Production 1.

[0120] FIG. 1(C) shows the C distribution of thin film negative electrode produced in Negative Electrode Production 1.BEST MODE FOR CARRYING OUT THE INVENTION

[0121] Embodiments of the invention will be explained below in detail. The following explanations on constituent elements are for embodiments (typical embodiments) of the invention, and should not be construed as limiting the invention unless they depart from the spirit of the invention.

[0122] The lithium secondary batteries of the invention each are a lithium secondary battery which comprises an electrode group constituted at least of a positive electrode, a negative electrode, and a microporous separator interposed between the electrodes and a nonaqueous electrolyte comprising a nonaqueous solvent and one or more lithium salts contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, and in which the positive electrode and the negative electrode each comprise a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing lithium ions, the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1) given above, fluorosilane compounds represented by general formula (2) given above, compounds represented by general formula (3) given above, compounds having an S—F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoricacid salts, difluorophosphoricacid salts, acetic acid salts, and propionic acid salts (hereinafter, these compounds are often abbreviated to “specific compounds”) in an amount of 10 ppm or more of the whole nonaqueous electrolyte, and the positive electrode, negative electrode, electrolyte, or battery constitution satisfies a specific requirement.

[0123] The lithium secondary batteries of the invention will be explained below in detail.<Positive Electrode>

[0124] The positive electrode for use in the lithium secondary batteries of the invention is explained below.

[0125] The positive electrode for use in the invention is not particularly limited as long as it has an active-material layer containing an active material capable of occluding and releasing lithium ions. However, it preferably is any positive electrode selected from the group consisting of the following positive electrode [1] to positive electrode [5]:positive electrode [1]: a positive electrode containing a positive-electrode active material containing manganese; positive electrode [2]: a positive electrode containing a positive-electrode active material having a composition represented by the following composition formula (4):LixNi(1−y−z)CoyMzO2  composition formula (4)[wherein M represents at least one element selected from the group consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; y represents a number satisfying 0.05≤y≤0.5; and z represents a number satisfying 0.01≤z≤0.5];positive electrode [3]: a positive electrode containing a positive-electrode active material selected from the group consisting of the following (a) to (d):(a) a positive-electrode active material having a BET specific surface area of from 0.4 m2 / g to 2 m2 / g,(b) a positive-electrode active material having an average primary-particle diameter of from 0.1 μm to 2 μm,(c) a positive-electrode active material having a median diameter d50 of from 1 μm to 20 μm,(d) a positive-electrode active material having a tap density of from 1.3 g / cm3 to 2.7 g / cm3;positive electrode [4]: a positive electrode satisfying any requirement selected from the group consisting of the following (e) to (f):(e) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material, a conductive material, and a binder on a current collector, wherein the content of the conductive material in the positive-electrode active-material layer is in the range of from 6% by mass to 20% by mass,(f) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the positive-electrode active-material layer has a density in the range of from 1.7 g / cm3 to 3.5 g / cm3,(g) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the ratio of the thickness of the positive-electrode active-material layer to the thickness of the current collector, i.e., the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector), is in the range of from 1.6 to 20; and positive electrode [5]: a positive electrode containing two or more positive-electrode active materials differing in composition.

[0127] Positive electrodes which are usually employed in the lithium secondary batteries of the invention will be first explained below.[Positive-Electrode Active Material]

[0128] The positive-electrode active material for general use in the positive electrodes will be described below.[[Composition]]

[0129] The positive-electrode active material is not particularly limited as long as it is capable of electrochemically occluding / releasing lithium ions. It preferably is a substance containing lithium and at least one transition metal. Examples thereof include lithium-transition metal composite oxides and lithium-containing transition metal / phosphoric acid compounds.

[0130] The transition metal in the lithium-transition metal composite oxides preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specific examples of the composite oxides include lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMnO2, LiMn2O4, and Li2MnO3, and ones formed by partly replacing the transition metal atom(s) as a main component of these lithium-transition metal composite oxides by one or more other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc. Examples of such compounds formed by replacement include LiNi0.5Mn0.33O2, LiNi0.85Co0.10Al0.05O2, LiNi0.33Co0.33Mn0.33O2, LiMn1.8Al0.2O4, and LiMn1.5Ni0.5O4.

[0131] The transition metal in the lithium-containing transition metal / phosphoric acid compounds preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specific examples of the compounds include iron phosphate compounds such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, cobalt phosphate compounds such as LiCoPO4, and ones formed by partly replacing the transition metal atom(s) as a main component of these lithium-transition metal / phosphoric acid compounds by one or more other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, Si, etc.[[Surface Coating]]

[0132] It is preferred to use a material comprising any of those positive-electrode active materials and, adherent to the surface thereof, a substance having a composition different from that of the core positive-electrode active material. Examples of the substance to be adhered to the surface (hereinafter abbreviated to “surface-adherent substance”) include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide, sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate, and carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate.

[0133] Those surface-adherent substances each can be adhered to the surface of a positive-electrode active material, for example, by: a method in which the substance is dissolved or suspended in a solvent and this solution or suspension is infiltrated into a positive-electrode active material and dried; a method in which a precursor for the surface-adherent substance is dissolved or suspended in a solvent and this solution or suspension is infiltrated into a positive-electrode active material and then heated or otherwise treated to react the precursor; or a method in which the substance is added to a precursor for a positive-electrode active material and heat-treated together with the precursor.

[0134] The amount of the surface-adherent substance to be used may be as follows. The lower limit thereof is preferably 0.1 ppm or larger, more preferably 1 ppm or larger, even more preferably 10 ppm or larger, in terms of mass ppm of the positive-electrode active material. The upper limit thereof is preferably 20% or smaller, more preferably 10% or smaller, even more preferably 5% or smaller, in terms of % by mass based on the positive-electrode active material. The surface-adherent substance serves to inhibit the nonaqueous electrolyte from undergoing an oxidation reaction on the surface of the positive-electrode active material, whereby the battery life can be improved. However, in case where the amount of the substance adhered is too small, that effect is not sufficiently produced. On the other hand, too large amounts thereof may result in an increase in resistance because the surface-adherent substance inhibits the occlusion / release of lithium ions.[[Shape]]

[0135] The particle shape of the positive-electrode active material to be used in the invention may be any of massive, polyhedral, spherical, ellipsoidal, platy, acicular, columnar, and other shapes such as those in common use. Preferred of these is one in which the primary particles have aggregated to form secondary particles and these secondary particles have a spherical or ellipsoidal shape. In electrochemical elements, the active material in each electrode usually expands / contracts with the charge / discharge of the element and, hence, deterioration is apt to occur, such as active-material breakage and conduction path breakage, due to the stress caused by the expansion / contraction. Because of this, a positive-electrode active material in which the primary particles have aggregated to form secondary particles is preferable to an active material composed of primary particles only since the particles in the former active material relieve the stress caused by expansion / contraction to prevent deterioration. Furthermore, particles of a spherical or ellipsoidal shape are preferable to particles showing axial orientation, e.g., platy ones, because the former particles are less apt to orient during electrode molding and hence this electrode is reduced in expansion / contraction during charge / discharge, and because these particles are apt to be evenly mixed with a conductive material in electrode production.[[Tap Density]]

[0136] The tap density of the positive-electrode active material is generally 1.3 g / cm3 or higher, preferably 1.5 g / cm3 or higher, more preferably 1.6 g / cm3 or higher, most preferably 1.7 g / cm3 or higher. When the tap density of the positive-electrode active material is lower than the lower limit, there are cases where it is necessary to use a larger amount of a dispersion medium and larger amounts of a conductive material and a binder in forming a positive-electrode active-material layer and this results in a limited loading of the positive-electrode active material in the positive-electrode active-material layer and a limited battery capacity. By employing a composite oxide powder having a high tap density, a positive-electrode active-material layer having a high density can be formed. The higher the tap density, the more the positive-electrode active material is generally preferred. Although there is no particular upper limit thereon, too high tap densities may result in cases where the diffusion of lithium ions in the positive-electrode active-material layer through the nonaqueous electrolyte as a medium becomes a rate-determining stage and this is apt to reduce load characteristics. Consequently, the tap density of the positive-electrode active material is generally 2.9 g / cm3 or lower, preferably 2.7 g / cm3 or lower, more preferably 2.5 g / cm3 or lower.

[0137] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the capacity of the cell with the sample, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining a density from the resultant volume of the sample and the weight thereof. This density is defined as the tap density.[[Median Diameter d50]]

[0138] The median diameter d50 (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the particles of the positive-electrode active material is generally 0.1 μm or larger, preferably 0.5 μm or larger, more preferably 1 μm or larger, most preferably 3 μm or larger. The upper limit thereof is generally 20 μm or smaller, preferably 18 μm or smaller, more preferably 16 μm or smaller, most preferably 15 μm or smaller. When the median diameter thereof is smaller than the lower limit, there are cases where a product having a high tap density cannot be obtained. When the median diameter thereof exceeds the upper limit, lithium diffusion in the individual particles requires a longer time and this results in a decrease in battery performance. In addition, there are cases where such positive-electrode active-material particles, when used in producing a positive electrode for batteries, i.e., when the active material and other ingredients including a conductive material and a binder are slurried with a solvent and this slurry is applied in a thin-film form, pose a problem, for example, that streak lines generate. It is possible to further improve loading in positive-electrode production by mixing two or more positive-electrode active materials differing in median diameter d50.

[0139] Median diameter d50 in the invention is determined with a known laser diffraction / scattering type particle size distribution analyzer. In the case where LA-920, manufactured by HORIBA, Ltd., is used as a particle size distribution analyzer, a 0.1% by mass aqueous solution of sodium hexametaphosphate is used as a dispersion medium for the examination and a five-minute ultrasonic dispersing treatment is conducted before the particles are examined at a measuring refractive index set at 1.24.[[Average Primary-Particle Diameter]]

[0140] In the case where the primary particles have aggregated to form secondary particles, the average primary-particle diameter of this positive-electrode active material is generally 0.01 μm or larger, preferably 0.05 μm or larger, more preferably 0.08 μm or larger, most preferably 0.1 μm or larger, and is generally 3 μm or smaller, preferably 2 μm or smaller, more preferably 1 μm or smaller, most preferably 0.6 μm or smaller. In case where the average primary-particle diameter thereof exceeds the upper limit, spherical secondary particles are difficult to form and this adversely influences powder loading or results in a considerably reduced specific surface area. There may hence be a high possibility that battery performances such as output characteristics might decrease. Conversely, when the average primary-particle diameter thereof is smaller than the lower limit, crystal growth is usually insufficient and, hence, there are cases where this positive-electrode active material poses a problem, for example, that charge / discharge reversibility is poor.

[0141] Primary-particle diameter is determined through an examination with a scanning electron microscope (SEM). Specifically, arbitrarily selected 50 primary-particle images in a photograph having a magnification of 10,000 diameters each are examined for the length of the longest segment of a horizontal line which extends across the primary-particle image from one side to the other side of the boundary. These measured lengths are averaged, whereby the average value can be determined.[[BET Specific Surface Area]]

[0142] The BET specific surface area of the positive-electrode active material to be used in the lithium secondary batteries of the invention is generally 0.2 m2 / g or larger, preferably 0.3 m2 / g or larger, more preferably 0.4 m2 / g or larger. The upper limit thereof is generally 4.0 m2 / g or smaller, preferably 2.5 m2 / g or smaller, more preferably 1.5 m2 / g or smaller. In case where the BET specific surface area thereof is smaller than the lower limit of that range, battery performances are apt to decrease. In case where the BET specific surface area thereof exceeds the upper limit, a high tap density is difficult to obtain and this may pose a problem concerning applicability in forming a positive-electrode active material.

[0143] BET specific surface area is measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.). The specific surface area is defined as a value obtained by preliminarily drying a sample at 150° C. for 30 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.[[Production Processes]]

[0144] For producing the positive-electrode active material, processes in general use for producing inorganic compounds may be employed. Especially for producing spherical or ellipsoidal active materials, various techniques are usable. Examples thereof include: a method which comprises dissolving or pulverizing / dispersing a transition metal source, e.g., a transition metal nitrate or transition metal sulfate, optionally together with a source of other element(s) in a solvent, e.g., water, regulating the pH of the solution or dispersion with stirring to produce a spherical precursor, recovering and optionally drying the precursor, subsequently adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; a method which comprises dissolving or pulverizing / dispersing a transition metal source, e.g., a transition metal nitrate, transition metal sulfate, transition metal hydroxide, or transition metal oxide, optionally together with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; and a method which comprises dissolving or pulverizing / dispersing a transition metal source, e.g., a transition metal nitrate, transition metal sulfate, transition metal hydroxide, or transition metal oxide, together with an Li source, e.g., LiOH, Li2CO3, or LiNO3, and optionally with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, and heat-treating the precursor at a high temperature to obtain the active material.[Constitution of Positive Electrode]

[0145] The constitution of the positive electrode to be used in the invention will be described below.[[Electrode Structure and Production Process]]

[0146] The positive electrode to be used in the lithium secondary batteries of the invention is produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector. The production of the positive electrode with a positive-electrode active material can be conducted in an ordinary manner. Namely, a positive-electrode active material and a binder are mixed together by a dry process optionally together with a conductive material, thickener, etc. and this mixture is formed into a sheet and press-bonded to a positive-electrode current collector. Alternatively, those materials are dissolved or dispersed in a liquid medium to obtain a slurry and this slurry is applied to a positive-electrode current collector and dried. Thus, a positive-electrode active-material layer is formed on the current collector, whereby the positive electrode can be obtained.

[0147] The content of the positive-electrode active material in the positive-electrode active-material layer in the invention is generally 10% by mass or higher, preferably 30% by mass or higher, especially preferably 50% by mass or higher. The upper limit thereof is generally 99.9% by mass or lower, preferably 99% by mass or lower. When the content of the positive-electrode active-material particles in the positive-electrode active-material layer is too low, there are cases where an insufficient electrical capacity results. Conversely, when the content thereof is too high, there are cases where the positive electrode has an insufficient strength. One positive-electrode active material may be used alone, or any desired combination of two or more positive-electrode active materials differing in composition or powder properties may be used in any desired proportion.[[Compaction]]

[0148] It is preferred that the positive-electrode active-material layer obtained by coating fluid application and drying should be compacted with a hand press, roller press, or the like in order to heighten the loading density of the positive-electrode active material. The lower limit of the density of the positive-electrode active-material layer is preferably 1 g / cm3 or higher, more preferably 1.5 g / cm3 or higher, even more preferably 2 g / cm3 or higher. The upper limit thereof is preferably 4 g / cm3 or lower, more preferably 3.5 g / cm3 or lower, even more preferably 3 g / cm3 or lower. When the density thereof exceeds the upper limit of that range, the infiltration of a nonaqueous electrolyte into around the current collector / active material interface becomes insufficient and there are cases where charge / discharge characteristics especially at a high current density decrease. When the density thereof is lower than the lower limit, there are cases where electrical conductivity among the active-material particles decreases to increase battery resistance.[[Conductive Material]]

[0149] As the conductive material, a known conductive material can be used at will. Examples thereof include metallic materials such as copper and nickel; graphites such as natural graphites and artificial graphites; carbon blacks such as acetylene black; and carbon materials such as amorphous carbon, e.g., needle coke. One of these materials may be used alone, or any desired combination of two or more thereof in any desired proportion may be used.

[0150] The conductive material may be used so that it is incorporated in the positive-electrode active-material layer in an amount of generally 0.01% by mass or larger, preferably 0.1% by mass or larger, more preferably 1% by mass or larger, and that the upper limit of the content thereof is generally 50% by mass or lower, preferably 30% by mass or lower, more preferably 15% by mass or lower. When the content thereof is lower than the lower limit of that range, there are cases where electrical conductivity becomes insufficient. Conversely, when the content thereof exceeds the upper limit of that range, there are cases where battery capacity decreases.[[Binder]]

[0151] The binder to be used for producing the positive-electrode active-material layer is not particularly limited. In the case where the layer is to be formed through coating fluid application, any binder may be used as long as it is a material which is soluble or dispersible in the liquid medium for use in electrode production. Examples thereof include resinous polymers such as polyethylene, polypropylene, poly(ethylene terephthalate), poly(methyl methacrylate), aromatic polyamides, cellulose, and nitrocellulose; rubbery polymers such as SBR (styrene / butadiene rubbers), NBR (acrylonitrile / butadiene rubbers), fluororubbers, isoprene rubbers, butadiene rubbers, and ethylene / propylene rubbers; thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers or products of hydrogenation thereof, EPDM (ethylene / propylene / diene terpolymers),styrene / ethylene / butadiene / ethylenecopolymers, and styrene / isoprene / styrene block copolymers or products of hydrogenation thereof; flexible resinous polymers such as syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene / vinyl acetate copolymers, and propylene / α-olefin copolymers; fluorochemical polymers such as poly(vinylidene fluoride) (PVdF), polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and polytetrafluoroethylene / ethylene copolymers; and polymer compositions having the property of conducting alkali metal ions (especially lithium ions). One of these substances may be used alone, or any desired combination of two or more thereof in any desired proportion may be used.

[0152] The proportion of the binder in the positive-electrode active-material layer is generally 0.1% by mass or higher, preferably 1% by mass or higher, more preferably 3% by mass or higher. The upper limit thereof is generally 80% by mass or lower, preferably 60% by mass or lower, more preferably 40% by mass or lower, most preferably 10% by mass or lower. When the proportion of the binder is too low, there are cases where the positive-electrode active material cannot be sufficiently held and the positive electrode has an insufficient mechanical strength to impair battery performances such as cycle characteristics. On the other hand, too high proportions thereof may lead to a decrease in battery capacity or conductivity.[[Liquid Medium]]

[0153] The kind of the liquid medium to be used for forming a slurry is not particularly limited as long as it is a solvent in which the positive-electrode active material, conductive material, and binder and a thickener, which is used according to need, can be dissolved or dispersed. Either an aqueous solvent or an organic solvent may be used.

[0154] Examples of the aqueous medium include water and mixed solvents comprising an alcohol and water. Examples of the organic medium include aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide.

[0155] Especially when an aqueous medium is used, it is preferred to use a thickener and a latex of, e.g., a styrene / butadiene rubber (SBR) to prepare a slurry. A thickener is used generally for the purpose of regulating the slurry viscosity. The thickener is not particularly limited. Examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, poly (vinyl alcohol), oxidized starch, phosphorylated starch, casein, and salts of these. One of these thickeners may be used alone, or any desired combination of two or more thereof in any desired proportion may be used. In the case where such a thickener is further added, the proportion of the thickener to the active material is generally 0.1% by mass or higher, preferably 0.5% by mass or higher, more preferably 0.6% by mass or higher. The upper limit thereof is generally 5% by mass or lower, preferably 3% by mass or lower, more preferably 2% by mass or lower. When the proportion thereof is lower than the lower limit of that range, there are cases where applicability decreases considerably. Proportions thereof exceeding the upper limit result in a reduced proportion of the active material in the positive-electrode active-material layer, and this may pose a problem that battery capacity decreases and a problem that resistance among the particles of the positive-electrode active material increases.[[Current Collector]]

[0156] The material of the positive-electrode current collector is not particularly limited, and a known one can be used at will. Examples thereof include metallic materials such as aluminum, stainless steel, nickel-plated materials, titanium, and tantalum; and carbon materials such as carbon cloths and carbon papers. Of these, metallic materials are preferred. Especially preferred is aluminum.

[0157] In the case of a metallic material, examples of the shape of the current collector include metal foils, metal cylinders, metal coils, metal plates, thin metal films, expanded metals, punching metals, and metal foams. In the case of a carbon material, examples of the collector shape include carbon plates, thin carbon films, and carbon cylinders. Of these, a thin metal film is preferred. The thin film may be in a suitable mesh form. Although the thin film may have any desired thickness, the thickness thereof is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger, and is generally 1 mm or smaller, preferably 100 μm or smaller, more preferably 50 μm or smaller. When the thin film is thinner than the lower limit of that range, there are cases where this film is deficient in strength required of a current collector. Conversely, when the thin film is thicker than the upper limit of that range, there are cases where this film has impaired handleability.

[0158] The thickness ratio between the current collector and the positive-electrode active-material layer is not particularly limited. However, the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector) is preferably 150 or smaller, more preferably 20 or smaller, most preferably 10 or smaller. The lower limit thereof is preferably 0.1 or larger, more preferably 0.4 or larger, most preferably 1 or larger. When that ratio exceeds the upper limit of that range, there are cases where this current collector heats up due to Joule's heat during high-current-density charge / discharge. When that ratio decreases beyond the lower limit of that range, the proportion by volume of the current collector to the positive-electrode active material increases and this may reduce the capacity of the battery.[[Electrode Area]]

[0159] When the nonaqueous electrolyte of the invention is used, it is preferred that the area of the positive-electrode active-material layer relative to the outer surface area of the battery case should be large from the standpoint of attaining a high output and enhanced high-temperature stability. Specifically, the overall electrode area of the positive electrode is preferably at least 15 times, more preferably at least 40 times, the surface area of the case of the secondary battery. When the battery has a bottomed polygonal shape, the term “outer surface area of the case” means a total area calculated from the length, width, and thickness dimensions of that case part packed with power-generating elements which excludes the terminal projections. When the battery has a bottomed cylindrical shape, that term means a geometric surface area obtained through the approximation to a cylinder of that case part packed with power-generating elements which excludes the terminal projections. The term “overall electrode area of the positive electrode” means the geometric surface area of the positive-electrode mix layer facing the mix layer containing a negative-electrode active material. In a structure in which a positive-electrode mix layer has been formed on each side of a current collector foil, that term means the sum of the areas separately calculated respectively for both sides.[[Discharge Capacity]]

[0160] When the nonaqueous electrolyte for secondary battery of the invention is used, it is preferred that the battery elements held in each battery case of the secondary battery should have an electric capacity (electric capacity measured when the battery in a fully charged state is discharged to a discharged state) of 3 ampere-hours (Ah) or higher. This is because such electric capacity is highly effective in improving low-temperature discharge characteristics.

[0161] For attaining that, a positive-electrode plate is designed to have a discharge capacity as measured in discharge from a fully charged state of preferably from 3 ampere-hours (Ah) to less than 20 Ah, more preferably from 4 Ah to less than 10 Ah. When the discharge capacity thereof is lower than 3 Ah, there are cases where high-current discharge results in a large decrease in voltage due to electrode reaction resistance and hence in a poor power efficiency. When the discharge capacity thereof is 20 Ah or higher, electrode reaction resistance is low and a satisfactory power efficiency is obtained. In this case, however, the battery has a wide temperature distribution during pulse charge / discharge due to internal heat generation and has poor durability in repetitions of charge / discharge. In addition, the efficiency of heat dissipation in the case of abrupt heat generation in an abnormality, such as overcharge or internal short-circuiting, is also poor. There are hence cases where the probability that the poor heat dissipation efficiency leads to a phenomenon in which the internal pressure increases and the gas release valve works (valve working) or a phenomenon in which the battery contents are vigorously ejected outside (rupture) increases.[[Thickness of Positive-Electrode Plate]]

[0162] The thickness of the positive-electrode plate is not particularly limited. However, from the standpoints of high capacity and high output, the thickness of the mix layer, excluding the thickness of the core metal foil, on one side of the current collector is such that the lower limit thereof is preferably 10 μm or larger, more preferably 20 μm or larger, and the upper limit thereof is preferably 200 μm or smaller, more preferably 100 μm or smaller.<Positive Electrode [1]>

[0163] Positive electrode [1], i.e., “a positive electrode containing a positive-electrode active material containing manganese”, for use in a lithium secondary battery of the invention will be explained below.

[0164] [Positive Electrode [1]: Positive-Electrode Active Material] The positive-electrode active material for use in positive electrode [1] is described below.[[Composition]]

[0165] The positive-electrode active material is a substance which contains one or more transition metals and is capable of electrochemically occluding / releasing lithium ions. One containing manganese as at least part of the transition metals is used in this invention. It is preferred that lithium should be further contained. More preferred is a composite oxide containing lithium and manganese.

[0166] The manganese-containing positive-electrode active material is not particularly limited. However, it preferably is one having a composition represented by the following composition formula (5):LixMn(1−y)M1yO2composition formula(5)[wherein M1 represents at least one element selected from the group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8].

[0167] In composition formula (5), M1 especially preferably is Ni, Co, or Fe; x especially preferably satisfies 0.2≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.7. Examples of the positive-electrode active material having a composition represented by composition formula (5) include LiMn0.5Ni0.5O2.

[0168] Especially preferred is one in which M1 is Ni and which has a composition represented by the following composition formula (6):LixMn(1−y−z)NiyM2zO2  composition formula (6)[wherein M2 represents at least one element selected from the group consisting of Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; y represents a number satisfying 0.05≤y≤0.8; and z represents a number satisfying 0.01≤z≤0.5]

[0169] In composition formula (6), M2 especially preferably is Co, Al, Fe, or Mg; x especially preferably satisfies 0.2≤x≤1.15; y especially preferably satisfies 0.1≤y≤0.7; z especially preferably satisfies 0.1≤z≤0.7; and y+z especially preferably satisfies 0.2≤y+z≤0.7. Examples of the positive-electrode active material having a composition represented by composition formula (6) include LiMn0.33Ni0.33Co0.33O2.

[0170] Also preferred as the manganese-containing positive-electrode active material is one having a composition represented by the following composition formula (7):LixMn(2−y)M3yO4  composition formula (7)[wherein M3 represents at least one element selected from the group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8].

[0171] In composition formula (7), M3especially preferably is Ni, Co, Al, or Mg; x especially preferably satisfies 0.05≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.7. Examples of the positive-electrode active material having a composition represented by composition formula (7) include LiMn1.8Al0.2O4 and LiMn1.5Ni0.5O4.[[Surface Coating]]

[0172] It is preferred to use an active material which comprises the manganese-containing positive-electrode active material and, adherent to the surface thereof, a substance having a composition different from that of the positive-electrode active material. The kind of the surface-adherent substance, method of adhesion, amount of the substance to be adherend, etc. are the same as those described above.[[Shape]]

[0173] The shape of the particles of this positive-electrode active material is the same as described above.[[Tap Density]]

[0174] The tap density of this positive-electrode active material is the same as described above.[[Median Diameter d50]]

[0175] The median diameter d50 (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the particles of this positive-electrode active material is the same as described above.[[Average Primary-Particle Diameter]]

[0176] In the case where the primary particles have aggregated to form secondary particles, the average primary-particle diameter of this positive-electrode active material is the same as described above.[[BET Specific Surface Area]]

[0177] The BET specific surface area of this positive-electrode active material is the same as described above.[[Production Processes]]

[0178] For producing this positive-electrode active material, processes in general use for producing inorganic compounds may be employed. Especially for producing the active material which is spherical or ellipsoidal, various techniques are usable. Examples thereof include: a method which comprises dissolving or pulverizing / dispersing a manganese source, e.g., manganese nitrate or manganese sulfate, optionally together with a source of other element(s) in a solvent, e.g., water, regulating the pH of the solution or dispersion with stirring to produce a spherical precursor, recovering and optionally drying the precursor, subsequently adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; a method which comprises dissolving or pulverizing / dispersing a manganese source, e.g., manganese nitrate, manganese sulfate, manganese oxide, or manganese oxyhydroxide, optionally together with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; and a method which comprises dissolving or pulverizing / dispersing a manganese source, e.g., manganese nitrate, manganese sulfate, manganese oxide, or manganese oxyhydroxide, together with an Li source, e.g., LiOH, Li2CO3, or LiNO3, and optionally with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, and heat-treating the precursor at a high temperature to obtain the active material.

[0179] In the invention, the manganese-containing positive-electrode active material may be used alone as the only positive-electrode active material, or a combination of the manganese-containing positive-electrode active material with any desired one or more positive-electrode active materials differing in composition or powder properties from the manganese-containing positive-electrode active material may be used in any desired proportion. In this case, preferred examples of the combination include: a combination of the manganese-containing positive-electrode active material with LiNiO2 or an active material formed by replacing part of the Ni by another transition metal; and a combination of the manganese-containing positive-electrode active material with LiCoO2 or an active material formed by replacing part of the Co by another transition metal. Especially preferred examples of the combination include: a combination of the positive-electrode active material represented by any of composition formulae (5) to (7) with LiNiO2 or an active material formed by replacing part of the Ni by another transition metal; and a combination of the positive-electrode active material represented by any of composition formulae (5) to (7) with LiCoO2 or an active material formed by replacing part of the Co by another transition metal. The content of the manganese-containing positive-electrode active material, in particular the positive-electrode active material represented by any of composition formulae (5) to (7), is preferably 30% by mass or higher, more preferably 50% by mass or higher, based on all positive-electrode active materials. When the positive-electrode active material represented by any of composition formulae (5) to (7) is used in too small proportion, there are cases where the cost of the positive electrode is not reduced.[Positive Electrode [1]: Constitution of Positive Electrode]

[0180] The constitution of the positive electrode to be used as positive electrode [1] is described below.

[0181] The electrode structure, process for electrode production, compaction of a positive-electrode active-material layer, conductive material, binder for use in producing a positive-electrode active-material layer, liquid medium for slurry formation, current collector, electrode area, discharge capacity, thickness of the positive-electrode plate, etc. in positive electrode [1] are the same as described above.<Positive Electrode [2]>

[0182] Positive electrode [2], i.e., “a positive electrode containing a positive-electrode active material having a specific composition represented by composition formula (4)”, for use in a lithium secondary battery of the invention will be explained below.[Positive Electrode [2]: Positive-Electrode Active Material]

[0183] The positive-electrode active material for use in positive electrode [2] is described below.[[Composition]]

[0184] As the positive-electrode active material is used a substance which contains one or more transition metals and is capable of electrochemically occluding / releasing lithium ions and which has a composition represented by the following composition formula (4) (hereinafter abbreviated to “this positive-electrode active material”):LixNi(1−y−z)CoyMzO2  composition formula (4)[wherein M represents at least one element selected from the group consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; y represents a number satisfying 0.05≤y≤0.5; and z represents a number satisfying 0.01≤z≤0.5]

[0185] In composition formula (4), M preferably is Mn, Al, or Mg, and x preferably satisfies 0.2≤x≤1.15. Symbol y preferably satisfies 0.08≤y≤0.4 and especially preferably satisfies 0.1≤y≤0.3. Furthermore, z preferably satisfies 0.02≤z≤0.4 and especially preferably satisfies 0.03≤z≤0.3.[[Surface Coating]]

[0186] It is preferred to use an active material which comprises this positive-electrode active material and, adherent to the surface thereof, a substance having a composition different from that of the positive-electrode active material. The kind of the surface-adherent substance, method of adhesion, amount of the substance to be adherend, etc. are the same as those described above.[[Shape]]

[0187] The shape of the particles of this positive-electrode active material is the same as described above.[[Tap Density]]

[0188] The tap density of this positive-electrode active material is the same as described above.[[Median Diameter d50]]

[0189] The median diameter d50 (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the particles of this positive-electrode active material is the same as described above.[[Average Primary-Particle Diameter]]

[0190] In the case where the primary particles have aggregated to form secondary particles, the average primary-particle diameter of this positive-electrode active material is the same as described above.[[BET Specific Surface Area]]

[0191] The BET specific surface area of this positive-electrode active material is the same as described above.[[Production Processes]]

[0192] For producing this positive-electrode active material, processes in general use for producing inorganic compounds may be employed. Especially for producing the active material which is spherical or ellipsoidal, various techniques are usable. Examples thereof include: a method which comprises dissolving or pulverizing / dispersing a nickel source, e.g., nickel nitrate or nickel sulfate, a cobalt source, e.g., cobalt nitrate or cobalt sulfate, and a source of M in composition formula (4) in a solvent, e.g., water, regulating the pH of the solution or dispersion with stirring to produce a spherical precursor, recovering and optionally drying the precursor, subsequently adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; a method which comprises dissolving or pulverizing / dispersing a nickel source, e.g., nickel nitrate, nickel sulfate, nickel oxide, nickel hydroxide, or nickel oxyhydroxide, a cobalt source, e.g., cobalt nitrate, cobalt sulfate, cobalt oxide, cobalt hydroxide, or cobalt oxyhydroxide, and a source of M in composition formula (4) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; and a method which comprises dissolving or pulverizing / dispersing a nickel source, e.g., nickel nitrate, nickel sulfate, nickel oxide, nickel hydroxide, or nickel oxyhydroxide, a cobalt source, e.g., cobalt nitrate, cobalt sulfate, cobalt oxide, cobalt hydroxide, or cobalt oxyhydroxide, a source of M in composition formula (4), and an Li source, e.g., LiOH, Li2CO3, or LiNO3 in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, and heat-treating the precursor at a high temperature to obtain the active material.

[0193] For producing the positive electrode in this invention, this positive-electrode active material (the positive-electrode active material represented by composition formula (4) and / or the positive-electrode active material represented by composition formula (4) covered with the surface-adherent substance) may be used alone, or a combination of this positive-electrode active material with any desired one or more positive-electrode active materials differing in composition from that positive-electrode active material may be used in any desired proportion. In this case, preferred examples of the combination include: a combination of this positive-electrode active material with LiMn2O4 or an active material formed by replacing part of the Mn by another transition metal, etc.; and a combination of this positive-electrode active material with LiCoO2 or an active material formed by replacing part of the Co with another transition metal, etc.

[0194] Here, the content of this positive-electrode active material is preferably 30% by mass or higher, more preferably 50% by mass or higher, based on all positive-electrode active materials. When this positive-electrode active material is used in too small a proportion, there are cases where the battery capacity decreases. Incidentally, “this positive-electrode active material” and “positive-electrode active materials other than this positive-electrode active material” are inclusively referred to as “positive-electrode active materials”.[Positive Electrode [2]: Constitution of Positive Electrode]

[0195] The constitution of the positive electrode to be used as positive electrode [2] is described below.

[0196] The electrode structure, process for electrode production, compaction of a positive-electrode active-material layer, conductive material, binder for use in producing a positive-electrode active-material layer, liquid medium for slurry formation, current collector, electrode area, discharge capacity, thickness of the positive-electrode plate, etc. in positive electrode [2] are the same as described above.<Positive Electrode [3]>

[0197] An explanation will be given below on positive electrode

[0198] for use in a lithium secondary battery of the invention, i.e., “a positive electrode containing a positive-electrode active material selected from the group consisting of the following (a) to (d):(a) a positive-electrode active material having a BET specific surface area of from 0.4 m2 / g to 2 m2 / g,(b) a positive-electrode active material having an average primary-particle diameter of from 0.1 μm to 2 μm,(c) a positive-electrode active material having a median diameter d50 of from 1 μm to 20 μm,(d) a positive-electrode active material having a tap density of from 1.3 g / cm3 to 2.7 g / cm3”.[Positive Electrode [3]: Positive-Electrode Active Material]

[0199] The positive-electrode active material for use in positive electrode [3] is described below.[[Composition]]

[0200] The positive-electrode active material is not particularly limited as long as it is a substance capable of electrochemically occluding / releasing lithium ions. It preferably is a substance containing lithium and at least one transition metal. Examples thereof include lithium-transition metal composite oxides and lithium-containing transition metal / phosphoric acid compounds. Specifically, such a compound having the same composition as described above may be used.[[Surface Coating]]

[0201] It is also preferred that the positive-electrode active material should comprise a core positive-electrode active material and, adherent to the surface thereof, a substance having a composition different from that of the core positive-electrode active material. The kind of the surface-adherent substance, method of adhesion, amount of the substance to be adhered, etc. are the same as those described above.[[Shape]]

[0202] The shape of the particles of this positive-electrode active material is the same as described above.[[BET Specific Surface Area]]

[0203] The BET specific surface area of this positive-electrode active material is preferably 0.4 m2 / g or larger, more preferably 0.5 m2 / g or larger, even more preferably 0.6 m2 / g or larger. The upper limit thereof is preferably 2 m2 / g or smaller, more preferably 1.8 m2 / g or smaller, even more preferably 1.5 m2 / g or smaller. In case where the BET specific surface area thereof is smaller than the lower limit of that range, battery performances are apt to decrease. In case where the BET specific surface area thereof exceeds the upper limit, a high tap density is difficult to obtain and this may pose a problem concerning applicability in forming a positive-electrode active material.

[0204] BET specific surface area is measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.). The specific surface area is defined as a value obtained by preliminarily drying a sample at 150° C. for 30 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.[[Average Primary-Particle Diameter]]

[0205] In the case where the primary particles have aggregated to form secondary particles, the average primary-particle diameter of this positive-electrode active material is preferably 0.1 μm or larger, more preferably 0.2 μm or larger, even more preferably 0.3 μm or larger, most preferably 0.4 μm or larger. The upper limit thereof is preferably 2 μm or smaller, more preferably 1.6 μm or smaller, even more preferably 1.3 μm or smaller, most preferably 1 μm or smaller. In case where the average primary-particle diameter thereof exceeds the upper limit, spherical secondary particles are difficult to form and this adversely influences powder loading or results in a considerably reduced specific surface area. There may hence be a high possibility that battery performances such as output characteristics might decrease. Conversely, when the average primary-particle diameter thereof is smaller than the lower limit, crystal growth is usually insufficient and, hence, there are cases where this positive-electrode active material poses a problem, for example, that charge / discharge reversibility is poor.

[0206] Primary-particle diameter is determined through an examination with a scanning electron microscope (SEM). Specifically, arbitrarily selected 50 primary-particle images in a photograph having a magnification of 10,000 diameters each are examined for the length of the longest segment of a horizontal line which extends across the primary-particle image from one side to the other side of the boundary. These measured lengths are averaged, whereby the average value can be determined.[[Median Diameter d50]]

[0207] The median diameter d50 (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the particles of the positive-electrode active material is preferably 1 μm or larger, more preferably 1.2 μm or larger, even more preferably 1.5 μm or larger, most preferably 2 μm or larger. The upper limit thereof is preferably 20 μm or smaller, more preferably 18 μm or smaller, even more preferably 16 μm or smaller, most preferably 15 μm or smaller. When the median diameter thereof is smaller than the lower limit, there are cases where a product having a high tap density cannot be obtained. When the median diameter thereof exceeds the upper limit, lithium diffusion in the individual particles requires a longer time and this results in a decrease in battery performance. In addition, there are cases where such positive-electrode active-material particles, when used in producing a positive electrode for batteries, i.e., when the active material and other ingredients including a conductive material and a binder are slurried with a solvent and this slurry is applied in a thin-film form, pose a problem, for example, that streak lines generate. The positive-electrode active material for use in this invention preferably is a mixture of two or more positive-electrode active materials differing in median diameter, because this mixture of positive-electrode active materials is effective in further improving loading in positive-electrode production.

[0208] Median diameter d50 in the invention is determined with a known laser diffraction / scattering type particle size distribution analyzer. In the case where LA-920, manufactured by HORIBA, Ltd., is used as a particle size distribution analyzer, a 0.1% by mass aqueous solution of sodium hexametaphosphate is used as a dispersion medium for the examination and a five-minute ultrasonic dispersing treatment is conducted before the particles are examined at a measuring refractive index set at 1.24.[[Tap Density]]

[0209] The tap density of this positive-electrode active material is preferably 1.3 g / cm3 or higher, more preferably 1.5 g / cm3 or higher, even more preferably 1.6 g / cm3 or higher, most preferably 1.7 g / cm3 or higher. When the tap density of the positive-electrode active material is lower than the lower limit, there are cases where it is necessary to use a larger amount of a dispersion medium and larger amounts of a conductive material and a binder in forming a positive-electrode active-material layer and this results in a limited loading of the positive-electrode active material in the positive-electrode active-material layer and a limited battery capacity. By employing a composite oxide powder having a high tap density, a positive-electrode active-material layer having a high density can be formed. The higher the tap density, the more the positive-electrode active material is generally preferred. Although there is no particular upper limit thereon, too high tap densities may result in cases where the diffusion of lithium ions in the positive-electrode active-material layer through the electrolyte as a medium becomes a rate-determining stage and this is apt to reduce load characteristics. Consequently, the upper limit of the tap density thereof is preferably 2.7 g / cm3 or lower, more preferably 2.5 g / cm3 or lower.

[0210] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the capacity of the cell with the sample, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining the resultant bulk density. This bulk density is defined as the tap density.

[0211] From the standpoints of long life and high output, this positive-electrode active material for use in a lithium secondary battery of the invention especially preferably is one which simultaneously satisfies two or more of the properties described above, i.e., BET specific surface area, average primary-particle diameter, median diameter d50, and tap density.[[Production Processes]]

[0212] Processes usable for producing this positive-electrode active material are the same as described above.

[0213] This positive-electrode active material for use in a lithium secondary battery of the invention may be a single positive-electrode active material alone, or any desired combination of two or more positive-electrode active materials differing in composition or powder properties may be used in any desired proportion as that positive-electrode active material. Furthermore, from the standpoint of life improvement, this positive electrode in the invention preferably contains a positive-electrode active material having any one of the properties described above (this positive-electrode active material) and a positive-electrode active material differing in composition from that positive-electrode active material.[Positive Electrode [3]: Constitution of Positive Electrode]

[0214] The constitution of the positive electrode to be used as positive electrode [3] is described below.

[0215] The electrode structure, process for electrode production, compaction of a positive-electrode active-material layer, conductive material, binder for use in producing a positive-electrode active-material layer, liquid medium for slurry formation, current collector, electrode area, discharge capacity, thickness of the positive-electrode plate, etc. in positive electrode [3] are the same as described above.<Positive Electrode [4]>

[0216] An explanation will be given below on positive electrode

[0217] for use in a lithium secondary battery of the invention, i.e., “a positive electrode satisfying any requirement selected from the group consisting of the following (e) to (f):(e) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material, a conductive material, and a binder on a current collector, wherein the content of the conductive material in the positive-electrode active-material layer is in the range of from 6% by mass to 20% by mass (embodiment 1),(f) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the positive-electrode active-material layer has a density in the range of from 1.7 g / cm3 to 3.5 g / cm3 (embodiment 2),(g) it is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the ratio of the thickness of the positive-electrode active-material layer to the thickness of the current collector, i.e., the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector), is in the range of from 1.6 to 20 (embodiment 3)”.[Positive Electrode [4]: Positive-Electrode Active Material]

[0218] The positive-electrode active material for use in positive electrode [4] is described below.

[0219] This positive-electrode active material is not particularly limited as long as it is a substance capable of electrochemically occluding / releasing lithium ions. It preferably is a substance containing lithium and at least one transition metal. Examples thereof include lithium-transition metal composite oxides and lithium-containing transition metal / phosphoric acid compounds. Specifically, such a compound having the same composition as described above may be used.

[0220] The surface coating, shape, tap density, median diameter d50, average primary-particle diameter, and BET specific surface area of this positive-electrode active material, processes for producing the active material, etc. are the same as described above.[Positive Electrode [4]: Constitution of Positive Electrode]

[0221] The constitution of the positive electrode to be used as positive electrode [4] is described below.[[Electrode Structure and Production Process]]

[0222] Positive electrode [4] in a lithium secondary battery of the invention is produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector. The production of this positive electrode with a positive-electrode active material can be conducted in an ordinary manner. Namely, at least a positive-electrode active material and a binder are mixed together by a dry process optionally together with a conductive material (essential ingredient in embodiment 1) and optionally further with a thickener, etc. and this mixture is formed into a sheet and press-bonded to a positive-electrode current collector. Alternatively, those materials are dissolved or dispersed in a liquid medium to obtain a slurry and this slurry is applied to a positive-electrode current collector and dried. Thus, a positive-electrode active-material layer is formed on the current collector, whereby the positive electrode can be obtained.

[0223] The content of the positive-electrode active material in the positive-electrode active-material layer is preferably 80% by mass or higher, more preferably 82% by mass or higher, especially preferably 84% by mass or higher. The upper limit thereof is preferably 95% by mass or lower, more preferably 93% by mass or lower. When the content of the positive-electrode active material in the positive-electrode active-material layer is too low, there are cases where an insufficient electric capacity results. Conversely, when the content thereof is too high, there are cases where the positive electrode has an insufficient strength.[[Conductive Material]]

[0224] As the conductive material, a known conductive material can be used at will. Examples thereof include the same conductive materials as shown above.

[0225] In this positive electrode for use in a lithium secondary battery of the invention, the content of the conductive material in the positive-electrode active-material layer in embodiment 1 should be 6% by mass or higher and is preferably 7% by mass or higher, especially preferably 8% by mass or higher, more preferably 9% by mass or higher. In embodiment 2 and embodiment 3, there are no particular limitations thereon. However, the content of the conductive material in embodiments 2 and 3 is preferably 6% by mass or higher, especially preferably 7% by mass or higher, more preferably 8% by mass or higher. When the content of the conductive material in the positive-electrode active-material layer is too low, there are cases where this positive-electrode active-material layer has insufficient electrical conductivity and a high output cannot be obtained.

[0226] The upper limit of the content of the conductive material in the positive-electrode active-material layer is as follows. In embodiment 1, the content thereof should be 20% by mass or lower and is preferably 18% by mass or lower, especially preferably 15% by mass or lower. In embodiment 2 and embodiment 3, the upper limit thereof is not particularly limited. However, the content of the conductive material in embodiments 2 and 3 is preferably 20% by mass or lower, especially preferably 18% by mass or lower, more preferably 15% by mass or lower. When the content thereof is too high, there are cases where battery capacity decreases and a high output cannot be obtained.[[Binder]]

[0227] The binder to be used for producing the positive-electrode active-material layer is not particularly limited. In the case where the layer is to be formed through coating fluid application, any binder may be used as long as it is a material which is soluble or dispersible in the liquid medium for use in electrode production. Examples thereof include the same binders as shown above.

[0228] The proportion of the binder in the positive-electrode active-material layer is generally 0.1% by mass or higher, preferably 1% by mass or higher, especially preferably 3% by mass or higher. The upper limit thereof is generally 80% by mass or lower, preferably 60% by mass or lower, especially preferably 40% by mass or lower, more preferably 10% by mass or lower. When the proportion of the binder is too low, there are cases where the positive-electrode active material cannot be sufficiently held and the positive electrode has an insufficient mechanical strength to impair battery performances such as cycle characteristics. On the other hand, too high proportions thereof may lead to a decrease in battery capacity or conductivity.[[Liquid Medium for Slurry Formation]]

[0229] The kind of the liquid medium to be used for forming a slurry is not particularly limited as long as it is a solvent in which the positive-electrode active material, conductive material, and binder and a thickener, which is used according to need, can be dissolved or dispersed. Either an aqueous solvent or an organic solvent may be used. Examples thereof include the same liquid media as shown above. The kind and addition amount of the thickener also are the same as described above.[[Compaction]]

[0230] It is preferred that the positive-electrode active-material layer obtained by coating fluid application and drying should be compacted with a handpress, roller press, or the like in order to heighten the loading density of the positive-electrode active material. The density of the positive-electrode active-material layer is as follows. In embodiment 2, the lower limit thereof should be 1.7 g / cm3 or higher and is preferably 2.0 g / cm3 or higher, especially preferably 2.2 g / cm3 or higher. In embodiment 1 and embodiment 3, there are no particular limitations. However, the lower limit thereof in embodiments 1 and 3 is preferably 1.7 g / cm3 or higher, especially preferably 2.0 g / cm3 or higher, more preferably 2.2 g / cm3 or higher. When the density of the layer is lower than the lower limit, there are cases where conductivity among the active-material particles decreases to increase battery resistance and a high output cannot be obtained.

[0231] The upper limit of the density of the positive-electrode active-material layer, in embodiment 2, should be 3.5 g / cm3 or lower and is preferably 3.0 g / cm3 or lower, especially preferably 2.8 g / cm3 or lower. In embodiment 1 and embodiment 3, the upper limit thereof is not particularly limited. However, the upper limit thereof in embodiments 1 and 3 is preferably 3.5 g / cm3 or lower, especially preferably 3.0 g / cm3 or lower, more preferably 2.8 g / cm3 or lower. When the density of the positive-electrode active-material layer exceeds the upper limit of that range, the infiltration of a nonaqueous electrolyte into around the current collector / active material interface decreases and there are cases where charge / discharge characteristics especially at a high current density decrease and a high output cannot be obtained.[[Current Collector]]

[0232] The material of the positive-electrode current collector is not particularly limited and is the same as described above. The shape of the current collector and the thickness of the thin film thereof also are the same as described above.

[0233] With respect to the ratio of the thickness of the positive-electrode active-material layer to that of the current collector, the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector) in embodiment 3 should be 20 or smaller and is preferably 15 or smaller, especially preferably 10 or smaller. In embodiment 1 and embodiment 2, that value is preferably 20 or smaller, especially preferably 15 or smaller, more preferably 10 or smaller, although there are no particular limitations thereon. When that ratio exceeds the upper limit of that range, there are cases where the current collector heats up due to Joule's heat during high-current-density charge / discharge to damage the positive electrode.

[0234] The lower limit of the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector) in embodiment 1 should be 1.6 or larger and is preferably 1.8 or larger, especially preferably 2 or larger. In embodiment 1 and embodiment 2, the lower limit thereof is preferably 1.6 or larger, especially preferably 1.8 or larger, more preferably 2 or larger, although there are no particular limitations thereon. When that ratio decreases beyond the lower limit, the proportion by volume of the current collector to the positive-electrode active material increases and there are hence cases where battery capacity decreases and a high output cannot be obtained.

[0235] When the preferred ranges of “the content of the conductive material in the positive-electrode active-material layer”, “the density of the positive-electrode active-material layer”, and “the ratio of the thickness of the positive-electrode active-material layer to that of the current collector, i.e., the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector)” are combined, then a more preferred high-output lithium secondary battery is obtained.[[Electrode Area]]

[0236] When the nonaqueous electrolyte of the invention is used, it is preferred that the area of the positive-electrode active-material layer relative to the outer surface area of the battery case should be large from the standpoint of attaining a high output and enhanced high-temperature stability. The electrode area is the same as described above.[[Discharge Capacity]]

[0237] When the nonaqueous electrolyte for secondary battery of the invention is used, it is preferred that the battery elements held in each battery case of the secondary battery should have an electric capacity (electric capacity measured when the battery in a fully charged state is discharged to a discharged state) of 3 ampere-hours (Ah) or higher. This is because such electric capacity is highly effective in improving output characteristics.

[0238] The design of a positive-electrode plate, etc. for attaining that electric capacity are the same as described above.[[Thickness of Positive-Electrode Plate]]

[0239] The thickness of the positive-electrode plate is not particularly limited, and is the same as described above.<Positive Electrode [5]>

[0240] Positive electrode [5] for use in a lithium secondary battery of the invention will be explained below.[Positive Electrode [5]: Positive-Electrode Active Materials]

[0241] The positive-electrode active materials for use in positive electrode [5] are described below.[[Composition]]

[0242] The positive-electrode active materials are not particularly limited as long as these are substances capable of electrochemically occluding / releasing lithium ions. They preferably are substances containing lithium and at least one transition metal. Examples thereof include lithium-transition metal composite oxides and lithium-containing transition metal / phosphoric acid compounds.

[0243] The transition metal in the lithium-transition metal composite oxides preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specific examples of the composite oxides include lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMnO2, LiMn2O4, and Li2MnO3, and ones formed by partly replacing the transition metal atom(s) as a main component of these lithium-transition metal composite oxides by one or more other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc.

[0244] The lithium-cobalt composite oxides are not particularly limited. However, one having a composition represented by the following composition formula (8) is preferred.LixCo(1−y)M1yO2  composition formula (8)[In composition formula (8), M1 represents at least one element selected from the group consisting of Ni, Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.055≤y≤0.8.]

[0245] In composition formula (8), M1 especially preferably is Ni, Mn, Al, or Fe; x especially preferably satisfies 0.2≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.5.

[0246] The lithium-nickel composite oxides are not particularly limited. However, one having a composition represented by the following composition formula (9) is preferred.LixNi(1−y)M2yO2  composition formula (9)[In composition formula (9), M2 represents at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8.]

[0247] In composition formula (9), M2 especially preferably is Co, Mn, Al, or Fe; x especially preferably satisfies 0.2≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.5.

[0248] The lithium-manganese composite oxides are not particularly limited. However, one having a composition represented by the following composition formula (10) is preferred.LixMn(1−y)M3yO2  composition formula (10)[In composition formula (10), M3 represents at least one element selected from the group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8.]

[0249] In composition formula (10), M1 especially preferably is Ni, Co, or Fe; x especially preferably satisfies 0.2≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.7.

[0250] Another preferred example of the lithium-manganese composite oxides is one having a composition represented by the following composition formula (11).LixMn(2−y)M4yO4  composition formula (11)[In composition formula (11), M4 represents at least one element selected from the group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8.]

[0251] In composition formula (11), M4 especially preferably is Ni, Co, Al, or Mg; x especially preferably satisfies 0.05≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.7.

[0252] Still another preferred example of the lithium-manganese composite oxides is one having a composition represented by the following composition formula (12).LixMn(1−y)M5yO3  composition formula (12)[In composition formula (12), M5 represents at least one element selected from the group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤2.4; and y represents a number satisfying 0.05≤y≤0.8.]

[0253] In composition formula (12), M5 especially preferably is Ni, Co, Al, or Mg; x especially preferably satisfies 0.1≤x≤2.3; and y especially preferably satisfies 0.1≤y≤0.5.

[0254] The transition metal in the lithium-containing transition metal / phosphoric acid compounds preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specific examples of the compounds include iron phosphate compounds such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, cobalt phosphate compounds such as LiCoPO4, and ones formed by partly replacing the transition metal atom(s) as a main component of these lithium-transition metal / phosphoric acid compounds by one or more other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, Si, etc.

[0255] The iron phosphate compounds are not particularly limited. However, one having a composition represented by the following composition formula (13) is preferred.LixFe(1−y)My6PO4  composition formula (13)[In composition formula (13), M6 represents at least one element selected from the group consisting of Ni, Co, Mn, Al, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; and y represents a number satisfying 0.05≤y≤0.8.]

[0256] In composition formula (13), M6 especially preferably is Ni, Co, Mn, or Al; x especially preferably satisfies 0.2≤x≤1.15; and y especially preferably satisfies 0.1≤y≤0.5.

[0257] It is preferred in this invention that any desired combination of two or more positive-electrode active materials differing in composition selected from the positive-electrode active materials described above should be used in any desired proportion. Those positive-electrode active materials differ from each other in merit performance and, hence, it is preferred to combine necessary performances according to the intended use of the battery. It is generally thought that a combination results in arithmetic-mean performances. With respect to life, however, there are cases where effects of the positive-electrode active material having a longer life are more produced unexpectedly. By using a positive-electrode active material attaining a relatively satisfactory life in combination with a positive-electrode active material which is relatively poor in life but is satisfactory in other performances, the lithium secondary battery of the invention having a high output, high capacity, and long life could be achieved.

[0258] Examples of such preferred combinations include: a positive-electrode active material represented by composition formula (8) and a positive-electrode active material represented by composition formula (10);

[0259] a positive-electrode active material represented by composition formula (8) and a positive-electrode active material represented by composition formula (11);

[0260] a positive-electrode active material represented by composition formula (8) and a positive-electrode active material represented by composition formula (12);

[0261] a positive-electrode active material represented by composition formula (8) and a positive-electrode active material represented by composition formula (13);

[0262] a positive-electrode active material represented by composition formula (9) and a positive-electrode active material represented by composition formula (10);

[0263] a positive-electrode active material represented by composition formula (9) and a positive-electrode active material represented by composition formula (11);

[0264] a positive-electrode active material represented by composition formula (9) and a positive-electrode active material represented by composition formula (12);

[0265] a positive-electrode active material represented by composition formula (9) and a positive-electrode active material represented by composition formula (13);

[0266] a positive-electrode active material represented by composition formula (10) and a positive-electrode active material represented by composition formula (11);

[0267] a positive-electrode active material represented by composition formula (10) and a positive-electrode active material represented by composition formula (12); and

[0268] a positive-electrode active material represented by composition formula (10) and a positive-electrode active material represented by composition formula (13).

[0269] Especially preferred are:

[0270] a positive-electrode active material represented by composition formula (9) and a positive-electrode active material represented by composition formula (11); and

[0271] a positive-electrode active material represented by composition formula (10) and a positive-electrode active material represented by composition formula (11).

[0272] The proportion in each combination is not particularly limited. However, it is preferably from 10:90 to 90:10, more preferably from 20:80 to 80:20.

[0273] Although two or more positive-electrode active materials differing in composition are contained in the positive electrode for use in the lithium secondary battery of this invention, it is preferred that at least one of these positive-electrode active materials should be one whose BET specific surface area, average primary-particle diameter, median diameter d50, and / or tap density (hereinafter abbreviated to “following properties”) is within the following specific range. The two or more positive-electrode active materials differing in composition may include a positive-electrode active material in which any property is not within the range shown below. It is, however, preferred that all of the two or more positive-electrode active materials should be ones in each of which any property is within the range shown below. The proportion of “all positive-electrode active materials in each of which any of the following properties is within the specific range shown below” to “all positive-electrode active materials contained in the positive electrode” depends on that property and is not particularly limited. However, that proportion is preferably 30% by mass or higher, more preferably 50% by mass or higher. Especially preferably, the former positive-electrode active materials account for the whole of the latter. It is preferred that each of the two or more positive-electrode active materials contained in the positive electrode should be one in which any one or more of the following properties are within the specific range (s) shown below. However, more preferably any two or more of, especially preferably any three or more of, most preferably all of the following properties of each positive-electrode active material are within the specific ranges shown below.[[BET Specific Surface Area]]

[0274] The BET specific surface area of at least one of the positive-electrode active materials is preferably 0.4 m2 / g or larger, more preferably 0.5 m2 / g or larger, even more preferably 0.6 m2 / g or larger. The upper limit thereof is 2 m2 / g or smaller, preferably 1.8 m2 / g or smaller, more preferably 1.5 m2 / g or smaller. In case where the BET specific surface area thereof is smaller than the lower limit of that range, battery performances are apt to decrease. In case where the BET specific surface area thereof exceeds the upper limit, a high tap density is difficult to obtain and this may pose a problem concerning applicability in forming a positive-electrode active material.

[0275] BET specific surface area is measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.). The specific surface area is defined as a value obtained by preliminarily drying a sample at 150° C. for 30 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.[[Average Primary-Particle Diameter]]

[0276] The average primary-particle diameter of at least one of the positive-electrode active materials is preferably 0.1 μm or larger, more preferably 0.2 μm or larger, even more preferably 0.3 μm or larger, most preferably 0.4 μm or larger. The upper limit thereof is preferably 2 μm or smaller, more preferably 1.6 μm or smaller, even more preferably 1.3 μm or smaller, most preferably 1 μm or smaller. In case where the average primary-particle diameter thereof exceeds the upper limit, spherical secondary particles are difficult to form and this adversely influences powder loading or results in a considerably reduced specific surface area. There may hence be a high possibility that battery performances such as output characteristics might decrease. Conversely, when the average primary-particle diameter thereof is smaller than the lower limit, crystal growth is usually insufficient and, hence, there are cases where this positive-electrode active material poses a problem, for example, that charge / discharge reversibility is poor.

[0277] Primary-particle diameter is determined through an examination with a scanning electron microscope (SEM). Specifically, arbitrarily selected 50 primary-particle images in a photograph having a magnification of 10,000 diameters each are examined for the length of the longest segment of a horizontal line which extends across the primary-particle image from one side to the other side of the boundary. These measured lengths are averaged, whereby the average value can be determined. There are cases where the primary particles have aggregated to form secondary particles. In such cases, however, the measurement is made only on primary particles.[[Median Diameter d50]]

[0278] The median diameter d50 (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the particles of at least one of the positive-electrode active materials is preferably 1 μm or larger, more preferably 1.2 μm or larger, even more preferably 1.5 μm or larger, most preferably 2 μm or larger. The upper limit thereof is preferably 20 μm or smaller, more preferably 18 μm or smaller, even more preferably 16 μm or smaller, most preferably 15 μm or smaller. When the median diameter thereof is smaller than the lower limit, there are cases where a product having a high tap density cannot be obtained. When the median diameter thereof exceeds the upper limit, lithium diffusion in the individual particles requires a longer time and this results in a decrease in battery performance. In addition, there are cases where such positive-electrode active-material particles, when used in producing a positive electrode for batteries, i.e., when the active material and other ingredients including a conductive material and a binder are slurried with a solvent and this slurry is applied in a thin-film form, pose a problem, for example, that streak lines generate. It is possible to further improve loading in positive-electrode production by mixing two or more positive-electrode active materials differing in median diameter d50.

[0279] Median diameter d50 in the invention is determined with a known laser diffraction / scattering type particle size distribution analyzer. In the case where LA-920, manufactured by HORIBA, Ltd., is used as a particle size distribution analyzer, a 0.1% by mass aqueous solution of sodium hexametaphosphate is used as a dispersion medium for the examination and a five-minute ultrasonic dispersing treatment is conducted before the particles are examined at a measuring refractive index set at 1.24.[[[Tap Density]]]

[0280] The tap density of at least one of the positive-electrode active materials is preferably 1.3 g / cm3 or higher, more preferably 1.5 g / cm3 or higher, even more preferably 1.6 g / cm3 or higher, most preferably 1.7 g / cm3 or higher. When the tap density of the positive-electrode active material is lower than the lower limit, there are cases where it is necessary to use a larger amount of a dispersion medium and larger amounts of a conductive material and a binder in forming a positive-electrode active-material layer and this results in a limited loading of the positive-electrode active material in the positive-electrode active-material layer and a limited battery capacity. By employing a composite oxide powder having a high tap density, a positive-electrode active-material layer having a high density can be formed. The higher the tap density, the more the positive-electrode active material is generally preferred. Although there is no particular upper limit thereon, too high tap densities may result in cases where the diffusion of lithium ions in the positive-electrode active-material layer through the nonaqueous electrolyte as a medium becomes a rate-determining stage and this is apt to reduce load characteristics. Consequently, the upper limit of the tap density thereof is preferably 2.7 g / cm3 or lower, more preferably 2.5 g / cm3 or lower.

[0281] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the capacity of the cell with the sample, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining the resultant bulk density. This bulk density is defined as the tap density.[[Surface Coating]]

[0282] It is preferred that at least one of the positive-electrode active materials should be one which has, adherent to the surface thereof, a substance having a composition different from that of the main positive-electrode active material or core positive-electrode active material (hereinafter, the substance adherent to the surface is abbreviated to “surface-adherent substance”). The kind of the surface-adherent substance, method of adhesion, amount of the substance to be adhered, etc. are the same as those described above.[[Shape]]

[0283] The shape of the particles of at least one of the positive-electrode active materials is the same as any of those of positive-electrode active-material particles heretofore in use.[[Production Processes]]

[0284] For producing the positive-electrode active materials, the same general processes for producing inorganic compounds as described above may be used.[Positive Electrode [5]: Constitution of Positive Electrode]

[0285] The constitution of positive electrode [5] for use in this invention is described below.

[0286] The electrode structure, process for electrode production, compaction of a positive-electrode active-material layer, conductive material, binder for use in producing a positive-electrode active-material layer, liquid medium for slurry formation, current collector, electrode area, discharge capacity, thickness of the positive-electrode plate, etc. in this invention are the same as described above. In producing the positive electrode, the two or more positive-electrode active materials may be mixed beforehand or may be mixed by simultaneously adding these during positive-electrode production.<Negative Electrode>

[0287] The negative electrode for use in the lithium secondary batteries of the invention is not particularly limited as long as it comprises a current collector and formed thereon an active-material layer containing an active material capable of occluding / releasing lithium ions. However, the negative electrode preferably is any negative electrode selected from the group consisting of the following negative electrode [1] to negative electrode

[10] :negative electrode [1]: a negative electrode containing as a negative-electrode active material two or more carbonaceous substances differing in crystallinity;negative electrode [2]: a negative electrode containing as a negative-electrode active material at least one amorphous carbonaceous substance which, when examined by wide-angle X-ray diffractometry, has an interplanar spacing (d002) for the (002) planes of 0.337 nm or larger and a crystallite size (Lc) of 80 nm or smaller and which, in an examination by argon ion laser Raman spectroscopy, has a Raman R value defined as the ratio of the peak intensity at 1,360 cm−1 to the peak intensity at 1,580 cm−1 of 0.2 or higher;negative electrode [3]: a negative electrode containing as a negative-electrode active material a titanium-containing metal oxide capable of occluding and releasing lithium;negative electrode [4]: a negative electrode containing as a negative-electrode active material a carbonaceous substance having a roundness of 0.85 or higher and a surface functional-group amount O / C value of from 0 to 0.01; negative electrode [5]: a negative electrode containing as a negative-electrode active material an orientation-differing-carbon composite comprising two or more carbonaceous substances differing in orientation;negative electrode [6]: a negative electrode containing, as a negative-electrode active material, graphitic carbon particles which have a roundness of 0.85 or higher and an interplanar spacing (d002) for the (002) planes as determined by wide-angle X-ray diffractometry of smaller than 0.337 nm and which, in an examination by argon ion laser Raman spectroscopy, have a Raman R value defined as the ratio of the peak intensity at 1,360 cm−1 to the peak intensity at 1,580 cm−1 of from 0.12 to 0.8; negative electrode [7]: a negative electrode containing as a negative-electrode active material a multielement-containing negative-electrode active material (C) which contains at least one of a lithium-occluding metal (A) selected from the group consisting of Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, and Sb and / or a lithium-occluding alloy (B) and further contains C and / or N as element Z;negative electrode [8]: a negative electrode containing as a negative-electrode active material two or more negative-electrode active materials differing in property; negative electrode [9]: a negative electrode containing a negative-electrode active material which has a tap density of 0.1 g / cm3 or higher and has a pore volume, in terms of the volume of particle pores corresponding to the diameter range of from 0.01 μm to 1 μm as determined by mercury porosimetry, of 0.01 mL / g or larger; andnegative electrode

[10] : a negative electrode which, when charged to 60% of a nominal capacity, has a reaction resistance in a negative-electrode opposing cell of 500 D or lower. Negative electrodes which are usually employed in the lithium secondary batteries of the invention will be first explained below.[Negative-Electrode Active Material]

[0288] The negative-electrode active material for general use in the negative electrodes will be explained below.[[Composition]]

[0289] The negative-electrode active material is not particularly limited as long as it is capable of electrochemically occluding / releasing lithium ions. Examples thereof include a carbonaceous material, a metal oxide such as tin oxide or silicon oxide, a metal composite oxide, elemental lithium, a lithium alloy such as a lithium-aluminum alloy, and a metal capable of alloying with lithium, such as Sn or Si. One of these may be used alone, or any desired two or more thereof may be used in combination in any desired proportion. From the standpoint of safety, it is preferred to use a carbonaceous material or a lithium composite oxide among those.

[0290] The metal composite oxide is not particularly limited as long as it is capable of occluding / releasing lithium. However, from the standpoint of high-current-density charge / discharge characteristics, it is preferred that the composite oxide should contain titanium and / or lithium as a component.

[0291] The carbonaceous material preferably is one selected from:(1) natural graphites;(2) artificial carbonaceous substances and artificial graphitic substances; and carbon materials obtained by subjecting carbonaceous substances [e.g., natural graphites, coal coke, petroleum coke, coal pitch, petroleum pitch, carbonaceous substances obtained by oxidizing these pitches, needle coke, pitch coke, carbon materials obtained by partly graphitizing these cokes, products of the pyrolysis of organic substances, such as furnace black, acetylene black, and pitch-derived carbon fibers, organic substances capable of carbonization (e.g., coal tar pitches ranging from soft pitch to hard pitch, coal-derived heavy oil such as dry distillation / liquefaction oil, straight-run heavy oil such as topping residues and vacuum distillation residues, heavy oils resulting from petroleum cracking, such as ethylene tar as a by-product of the thermal cracking of crude oil, naphtha, etc., aromatic hydrocarbons such as acenaphthylene, decacyclene, anthracene, and phenanthrene, nitrogen-ring compounds such as phenazine and acridine, sulfur-ring compounds such as thiophene and bithiophene, polyphenylenes such as biphenyl and terphenyl, poly(vinylchloride), poly(vinylalcohol),poly(vinylbutyral), substances obtained by insolubilizing these compounds, nitrogen-containing organic polymers such as polyacrylonitrile and polypyrrole, sulfur-containing organic polymers such as polythiophene, organic polymers such as polystyrene, natural polymers such as polysaccharides represented by cellulose, lignin, mannan, poly(galacturonic acid), chitosan, and saccharose, thermoplastic resins such as poly(phenylene sulfide) and poly(phenylene oxide), and thermosetting resins such as furfuryl alcohol resins, phenol-formaldehyde resins, and imide resins) and products of the carbonization thereof, or solutions obtained by dissolving any of such organic substances capable of carbonization in a low-molecular organic solvent, e.g., benzene, toluene, xylene, quinoline, or n-hexane, and products of the carbonization of these solutions] to a heat treatment at a temperature in the range of from 400° C. to 3,200° C. one or more times;(3) carbon materials constituting a negative-electrode active-material layer which comprise at least two carbonaceous substances differing in crystallinity and / or have an interface where at least two carbonaceous substances differing in crystallinity are in contact with each other; and(4) carbon materials constituting a negative-electrode active-material layer which comprise at least two carbonaceous substances differing in orientation and / or have an interface where at least two carbonaceous substances differing in orientation are in contact with each other. This is because this carbonaceous material brings about a satisfactory balance between initial irreversible capacity and high-current-density charge / discharge characteristics.[Constitution of Negative Electrode, Properties, and Method of Preparation]

[0292] With respect to the properties of the carbonaceous material, negative electrode containing the carbonaceous material, method of electrode formation, current collector, and lithium secondary battery, it is desirable that any one of the following (1) to (19) should be satisfied or two or more thereof be simultaneously satisfied.(1) X-Ray Parameter

[0293] The carbonaceous material preferably has a value of d (interplanar spacing) for the lattice planes (002), as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, of 0.335 nm or larger. It is desirable that the upper limit of the value of d should be generally 0.36 nm or smaller, preferably 0.35 nm or smaller, more preferably 0.345 nm or smaller. The crystallite size (Lc) of the carbonaceous material, as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is preferably 1 nm or larger, more preferably 1.5 nm or larger.(2) Ash Content

[0294] The ash content of the carbonaceous material may be 1% by mass or lower and is especially 0.5% by mass or lower, in particular 0.1% by mass or lower, based on the whole carbonaceous material. The lower limit of the ash content thereof is preferably at least 1 ppm by mass of the whole carbonaceous material. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the nonaqueous electrolyte during charge / discharge becomes not negligible. When the ash content thereof is lower than the lower limit of that range, there are cases where the production of this carbonaceous material necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.(3) Volume-Average Particle Diameter

[0295] With respect to the volume-average particle diameter of the carbonaceous material, the volume-average particle diameter (median diameter) thereof as determined by the laser diffraction / scattering method is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger, even more preferably 7 μm or larger. The upper limit thereof is generally 100 μm or smaller, preferably 50 μm or smaller, more preferably 40 μm or smaller, even more preferably 30 μm or smaller, especially preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where irreversible capacity increases, leading to a loss in initial battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such a carbonaceous material is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode is produced through coating fluid application.

[0296] Volume-average particle diameter in the invention is defined as the median diameter determined by dispersing the carbon powder in a 0.2% by mass aqueous solution (about 10 mL) of poly (oxyethylene (20)) sorbitan monolaurate as a surfactant and examining the dispersion with a laser diffraction / scattering type particle size distribution analyzer (e.g., LA-700, manufactured by HORIBA, Ltd.).(4) Raman R Value, Raman Half-Value Width

[0297] The R value of the carbonaceous material as determined by the argon ion laser Raman spectroscopy is generally 0.01 or higher, preferably 0.03 or higher, more preferably 0.1 or higher. The upper limit thereof may be 1.5 or lower and is preferably 1.2 or lower, more preferably 1.0 or lower, even more preferably 0.5 or lower. When the R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the R value thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the nonaqueous electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0298] The Raman half-value width around 1,580 cm−1 of the carbonaceous material is not particularly limited. However, the half-value width thereof is generally 10 cm−1 or larger, preferably 15 cm−1 or larger. The upper limit thereof is generally 100 cm−1 or smaller, preferably 80 cm−1 or smaller, more preferably 60 cm−1 or smaller, even more preferably 40 cm−1 or smaller. When the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the nonaqueous electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0299] The examination for a Raman spectrum is made with a Raman spectrometer (e.g., a Raman spectrometer manufactured by Japan Spectroscopic Co., Ltd.). In the examination, a sample is charged into a measuring cell by causing the sample to fall naturally into the cell and the surface of the sample in the cell is irradiated with argon ion laser light while rotating the cell in a plane perpendicular to the laser light. The Raman spectrum obtained is examined for the intensity IA of a peak PA around 1,580 cm−1 and the intensity IB of a peak PB around 1,360 cm−1. The ratio between these intensities R (R=IB / IA) is calculated and this ratio is defined as the Raman R value of the carbonaceous material. Furthermore, the half-value width of the peak PA around 1,580 cm−1 in the Raman spectrum obtained is measured, and this value is defined as the Raman half-value width of the carbonaceous material.

[0300] Conditions for this Raman spectroscopy are as follows.

[0301] Wavelength of argon ion laser: 514.5 nm

[0302] Laser power on sample: 15-25 mW

[0303] Resolution: 10-20 cm1

[0304] Examination range: 1,100 cm−1 to 1,730 cm−1

[0305] Analysis for R value and half-value width: background processing

[0306] Smoothing: simple average; convolution, 5 points(5) BET Specific Surface Area

[0307] The specific surface area of the carbonaceous material for use in the invention, as determined by the BET method, is generally 0.1 m2 / g or larger, preferably 0.7 m2 / g or larger, more preferably 1.0 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is generally 100 m2 / g or smaller, preferably 25 m2 / g or smaller, more preferably 15 m2 / g or smaller, even more preferably 10 m2 / g or smaller. When the specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this carbonaceous material as a negative-electrode material is apt to result in poor lithium acceptance during charge and in lithium deposition on the electrode surface. On the other hand, when the specific surface area thereof exceeds the upper limit of that range, there are cases where use of this carbonaceous material as a negative-electrode material is apt to result in enhanced reactivity with the nonaqueous electrolyte and enhanced gas evolution and a preferred battery is difficult to obtain.

[0308] The specific surface area as determined by the BET method is a value measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by preliminarily drying a sample at 350° C. for 15 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.(6) Pore Diameter Distribution

[0309] With respect to the pore diameter distribution of the carbonaceous material for use in the invention, the amount of interstices corresponding to a pore diameter of from 0.01 μm to 1 μm which include pores in the particles, particle surface irregularities formed by steps, and contact surfaces among the particles, as determined by mercury porosiometry (mercury intrusion method), may be 0.01 mL / g or larger and is preferably 0.05 mL / g or larger, more preferably 0.1 mL / g or larger. The upper limit thereof may be 0.6 mL / g or smaller and is preferably 0.4 mL / g or smaller, more preferably 0.3 mL / g or smaller. When the amount of interstices therein is larger than the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the amount thereof is smaller than the lower limit, there are cases where high-current-density charge / discharge characteristics decrease and the effect of diminishing electrode expansion / contraction during charge / discharge is not obtained.

[0310] The total volume of pores corresponding to the pore diameter range of from 0.01 μm to 100 μm is preferably 0.1 mL / g or larger, more preferably 0.25 mL / g or larger, even more preferably 0.4 mL / g or larger. The upper limit thereof may be 10 mL / g or smaller and is preferably 5 mL / g or smaller, more preferably 2 mL / g or smaller. When the total volume thereof exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the total volume thereof is smaller than the lower limit, there are cases where the dispersing effect of a thickener or binder in electrode plate formation is not obtained.

[0311] The average pore diameter of the carbonaceous material is preferably 0.05 μm or larger, more preferably 0.1 μm or larger, even more preferably 0.5 μm or larger. The upper limit thereof may be 50 μm or smaller and is preferably 20 μm or smaller, more preferably 10 μm or smaller. When the average pore diameter thereof exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary. When the average pore diameter thereof is smaller than the lower limit, there are cases where high-current-density charge / discharge characteristics decrease.

[0312] As an apparatus for the mercury porosimetry is used a mercury porosimeter (Autopore 9520, manufactured by Micromeritics Corp.). About 0.2 g of a sample is placed in a powder cell and this cell is closed. The sample in the cell is pretreated by degassing it at room temperature under vacuum (50 μmHg or lower) for 10 minutes. Subsequently, the pressure in the cell is reduced to 4 psia (about 28 kPa) and mercury is introduced thereinto. The internal pressure is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia (about 280 MPa) and then lowered to 25 psia (about 170 kPa). The number of steps in the pressure elevation is 80 or larger. In each step, the amount of mercury intruded is measured after an equilibrium time of 10 seconds. A pore diameter distribution is calculated from the mercury intrusion curve thus obtained, using the Washburn equation. Incidentally, the surface tension (γ) and contact angle (ψ) of mercury are taken as 485 dyne / cm and 140°, respectively. The average pore diameter used is the pore diameter corresponding to a cumulative pore volume of 50%.(7) Roundness

[0313] Roundness is used as an index to the degree of sphericity of the carbonaceous material. The roundness of the particles having a particle diameter in the range of 3-40 μm is preferably 0.1 or higher, especially preferably 0.5 or higher, more preferably 0.8 or higher, even more preferably 0.85 or higher, most preferably 0.9 or higher. High roundness values are preferred because they are effective in improving high-current-density charge / discharge characteristics.

[0314] Roundness is defined by the following equation. When a particle has a roundness of 1, this particle theoretically is a true sphere.Roundness=(length of periphery of equivalent circle having the same area as projected particle shape) / (actual length of periphery of projected particle shape)

[0315] The value of roundness is determined, for example, with a flow type particle image analyzer (e.g., FPIA, manufactured by Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene (20)) sorbitan monolaurate as a surfactant, propagating an ultrasonic wave of 28 kHz to the dispersion for 1 minute at an output of 60 W, and then examining particles having a particle diameter in the range of 3-40 μm with the analyzer having a detection range set at 0.6-400 μm.

[0316] Methods for improving roundness are not particularly limited. However, a carbonaceous material in which the particles have been rounded by a rounding treatment (mechanical energy treatment) is preferred because it gives an electrode in which the interstices among particles are uniform in shape. Examples of the rounding treatment include: a method in which a shear force or compressive force is applied to thereby mechanically make the shape of the particles close to sphere; and a method of mechanical / physical treatment in which fine particles are aggregated into particles by means of the bonding force of a binder or of the fine particles themselves.(8) True Density

[0317] The true density of the carbonaceous material is generally 1.4 g / cm3 or higher, preferably 1.6 g / cm3 or higher, more preferably 1.8 g / cm3 or higher, even more preferably 2.0 g / cm3 or higher. The upper limit thereof may be 2.26 g / cm3 or lower. This upper limit is a theoretical value for graphites. When the true density of the carbonaceous material is lower than the lower limit of that range, there are cases where this carbon has too low crystallinity, resulting in an increase in initial irreversible capacity. In the invention, true density is defined as one determined by the liquid-phase displacement method (pycnometer method) using butanol.(9) Tap Density

[0318] The tap density of the carbonaceous material desirably is generally 0.1 g / cm3 or higher, preferably 0.5 g / cm3 or higher, more preferably 0.7 g / cm3 or higher, especially preferably 1.0 g / cm3 or higher. The upper limit thereof is preferably 2 g / cm3 or lower, more preferably 1.8 g / cm3 or lower, especially preferably 1.6 g / cm3 or lower. When the tap density thereof is lower than the lower limit of that range, there are cases where this carbonaceous material, when used in a negative electrode, is less apt to have a high loading density and cannot give a battery having a high capacity. On the other hand, when the tap density thereof exceeds the upper limit of that range, the amount of interparticle interstices in the electrode is too small and it is difficult to secure electrical conductivity among the particles. There are hence cases where preferred battery performances are difficult to obtain. Tap density is determined by the same method as described above with regard to the positive electrode and is defined as described therein.(10) Orientation Ratio

[0319] The orientation ratio of the carbonaceous material is generally 0.005 or higher, preferably 0.01 or higher, more preferably 0.015 or higher. The upper limit thereof is theoretically 0.67 or lower. When the orientation ratio thereof is lower than the lower limit of that range, there are cases where high-density charge / discharge characteristics decrease.

[0320] Orientation ratio is determined by X-ray diffractometry after a sample is molded by compaction. A molding obtained by packing 0.47 g of a sample into a molding machine having a diameter of 17 mm and compacting the sample at 600 kgf / cm2 is set with clay on a sample holder for examination so as to be flush with the holder. This sample molding is examined for X-ray diffraction. From the intensities of the resultant (110) diffraction peak and (004) diffraction peak for the carbon, the ratio represented by (110) diffraction peak intensity / (004) diffraction peak intensity is calculated. This ratio is defined as the orientation ratio of the active material.

[0321] Conditions for this X-ray diffractometry are as follows. Incidentally, “2θ” represents diffraction angle.

[0322] Target: Cu(Kα line) graphite monochromator

[0323] Slit:

[0324] Divergence slit=0.5 degrees

[0325] Receiving slit=0.15 mm

[0326] Scattering slit=0.5 degrees

[0327] Examination range and step angle / measuring time:

[0328] (110) plane: 75°≤2θ 80° 1° / 60 sec

[0329] (004) plane: 52°≤2θ≤57° 1° / 60 sec(11) Aspect Ratio (Powder)

[0330] The aspect ratio is theoretically 1 or higher. The upper limit thereof may be 10 or lower and is preferably 8 or lower, more preferably 5 or lower. When the aspect ratio exceeds the upper limit, there are cases where this carbonaceous material causes streak lines in electrode plate formation and an even coating surface cannot be obtained, resulting in a decrease in high-current-density charge / discharge characteristics.

[0331] The aspect ratio of a carbonaceous-material particle is represented by A / B, wherein A is the length of the longest axis of the particle in a three-dimensional examination and B is the length of the shortest axis perpendicular to that axis. The examination of carbon particles is made with a scanning electron microscope capable of enlargement examination. Fifty are arbitrarily selected from graphite particles fixed to an edge face of a metal sheet having a thickness of 50 μm or smaller, and each particle is examined for A and B while rotating or inclining the stage to which the sample is fixed. The average of the A / B values is determined.(12) Minor-Material Mixing

[0332] “Minor-material mixing” means that the negative electrode and / or the negative-electrode active material contains two or more carbonaceous materials differing in property. The term “property” herein means one or more properties selected from X-ray diffraction parameter, median diameter, aspect ratio, BET specific surface area, orientation ratio, Raman R value, tap density, true density, pore distribution, roundness, and ash content.

[0333] Especially preferred embodiments include: one in which the volume-based particle size distribution is not symmetrical about the median diameter; one in which two or more carbonaceous materials differing in Raman R value are contained; and one in which carbonaceous materials differing in X-ray parameter are contained.

[0334] One example of the effects thereof is that the incorporation of a carbonaceous material, such as a graphite, e.g., a natural graphite or artificial graphite, or an amorphous carbon, e.g., a carbon black such as acetylene black or needle coke, as a conductive material serves to reduce electrical resistance. One of these carbonaceous materials may be used alone, or any desired two or more thereof may be used in combination in any desired proportion. In the case where such a carbonaceous material is added as a conductive material, the amount thereof may be 0.1% by mass or larger and is preferably 0.5% by mass or larger, more preferably 0.6% by mass or larger. The upper limit thereof may be 45% by mass or smaller and is preferably 40% by mass. Amounts thereof smaller than the lower limit of that range may result in difficulties in obtaining the effect of improving conductivity. Amounts thereof exceeding the upper limit may lead to an increase in initial irreversible capacity.(13) Electrode Production

[0335] Electrode production may be conducted by an ordinary method. For example, a binder and a solvent are added to a negative-electrode active material optionally together with a thickener, conductive material, filler, etc. to obtain a slurry and this slurry is applied to a current collector and dried. Thereafter, the coated current collector is pressed, whereby an electrode can be formed. The thickness of the negative-electrode active-material layer per one side in the stage just before the step of injecting a nonaqueous electrolyte in battery fabrication is generally 15 μm or larger, preferably m or larger, more preferably 30 μm or larger. The upper limit thereof may be 150 μm or smaller and is preferably 120 μm or smaller, more preferably 100 μm or smaller. When the thickness thereof is larger than the upper limit of that range, a nonaqueous electrolyte is less apt to infiltrate into around the interface of the current collector and, hence, there are cases where high-current-density charge / discharge characteristics decrease. When the thickness thereof is smaller than the lower limit of that range, the proportion by volume of the current collector to the negative-electrode active material increases and there are cases where battery capacity decreases. The negative-electrode active material may be roller pressed to obtain a sheet electrode. It is also possible to subject the negative-electrode active material to compression molding to obtain a pellet electrode.(14) Current Collector

[0336] As the current collector, a known one can be used at will. Examples of the current collector for the negative electrode include metallic materials such as copper, nickel, stainless steel, and nickel-plated steel. Copper is especially preferred of these from the standpoints of processability and cost. In the case where the current collector is a metallic material, examples of the shape of the current collector include metal foils, metal cylinders, metal coils, metal plates, thin metal films, expanded metals, punching metals, and metal foams. Preferred of these are thin metal films. More preferred are copper foils. Even more preferred are a rolled copper foil, which is produced by the rolling process, and an electrolytic copper foil, which is produced by the electrolytic process. Either of these can be used as a current collector. In the case of a copper foil having a thickness smaller than 25 m, use can be made of a copper alloy (e.g., phosphor bronze, titanium-copper, Corson alloy, or Cu—Cr—Zr alloy) having a higher strength than pure copper. Furthermore, an aluminum foil can be advantageously used because it has a low specific gravity and, hence, use of the foil as a current collector can reduce the weight of the battery.

[0337] The current collector comprising a copper foil produced by the rolling process is less apt to crack even when the negative electrode is rolled tightly or rolled at an acute angle, because the copper crystals are oriented in the rolling direction. This current collector can be advantageously used in small cylindrical batteries. The electrolytic copper foil is obtained by immersing a metallic drum in a nonaqueous electrolyte containing copper ions dissolved therein, causing current to flow through the system while rotating the drum to thereby deposit copper on the drum surface, and peeling the copper deposit from the drum. Copper may be deposited on a surface of the rolled copper foil by the electrolytic process. One or each side of such a copper foil may have undergone a surface-roughening treatment or a surface treatment (e.g., a chromate treatment in a thickness of from several nanometers to about 1 μm or a priming treatment with titanium).

[0338] The current collector base is desired to further have the following properties.

[0339] (1) Average Surface Roughness (Ra)

[0340] The average surface roughness (Ra) of that side of the current collector base on which a thin active-material film is to be formed, as determined by the method provided for in JIS B0601-1994, is not particularly limited. However, the average surface roughness thereof is generally 0.05 μm or higher, preferably 0.1 μm or higher, especially preferably 0.15 μm or higher. The upper limit thereof is generally 1.5 μm or lower, preferably 1.3 μm or lower, especially preferably 1.0 μm or lower. By regulating the average surface roughness (Ra) of the current collector base so as to be in the range of from the lower limit to upper limit, satisfactory charge / discharge cycle characteristics can be expected. By regulating the average surface roughness thereof to a value not lower than the lower limit, the area of the interface between the base and a thin active-material film is increased and adhesion to the thin active-material film is improved. The upper limit of the average surface roughness (Ra) thereof is not particularly limited. However, it is preferably 1.5 μm or lower because a foil having a practical thickness for batteries and having an average surface roughness (Ra) exceeding 1.5 μm is generally difficult to procure.

[0341] (2) Tensile Strength

[0342] The tensile strength of the current collector base is not particularly limited. However, it is generally 100 N / mm2 or higher, preferably 250 N / mm2 or higher, more preferably 400 N / mm2 or higher, especially preferably 500 N / mm2 or higher. Tensile strength is a quotient obtained by dividing the maximum tensile force required before test piece breakage by the sectional area of the test piece. In the invention, the tensile strength is determined through a measurement conducted with the same apparatus and by the same method as for elongation. A current collector base having a high tensile strength can be inhibited from cracking with the expansion / contraction of the thin active-material film which occur upon charge / discharge. With this current collector base, satisfactory cycle characteristics can be obtained.

[0343] (3) 0.2% Proof Stress

[0344] The 0.2% proof stress of the current collector base is not particularly limited. However, it is generally 30 N / mm2 or higher, preferably 150 N / mm2 or higher, especially preferably 300 N / mm2 or higher. The term 0.2% proof stress means the degree of load necessary for imparting a plastic (permanent) deformation of 0.2%. Namely, it means that application of that degree of load and subsequent removal thereof result in a 0.2% deformation. In the invention, the 0.2% proof stress is determined through a measurement conducted with the same apparatus and by the same method as for elongation. A current collector base having a high 0.2% proof stress can be inhibited from plastically deforming with the expansion / contraction of the thin active-material film which occur upon charge / discharge. With this current collector base, satisfactory cycle characteristics can be obtained.

[0345] The thin metal film may have any desired thickness. However, the thickness thereof is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger. The upper limit thereof is generally 1 mm or smaller, preferably 100 μm or smaller, more preferably 30 μm or smaller. In case where the thin metal film is thinner than 1 μm, this film has a reduced strength and there are hence cases where coating is difficult. When the thin metal film is thicker than 100 μm, there are cases where this film deforms an electrode shape, e.g., a rolled form. The thin metal film may be in a mesh form.(15) Thickness Ratio between Current Collector andActive-Material Layer

[0346] The thickness ratio between the current collector and the active-material layer is not particularly limited. However, the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector) is preferably 150 or smaller, especially preferably 20 or smaller, more preferably 10 or smaller. The lower limit thereof is preferably 0.1 or larger, especially preferably 0.4 or larger, more preferably 1 or larger. When that ratio exceeds the upper limit of that range, there are cases where this current collector heats up due to Joule's heat during high-current-density charge / discharge. When that ratio decreases beyond the lower limit of that range, the proportion by volume of the current collector to the negative-electrode active material increases and this may reduce the capacity of the battery.(16) Electrode Density

[0347] When the negative-electrode active material is used to form an electrode, the electrode structure is not particularly limited. However, the density of the active material present on the current collector is preferably 1.0 g / cm3 or higher, more preferably 1.2 g / cm3 or higher, even more preferably 1.3 g / cm3 or higher. The upper limit thereof is generally 2.0 g / cm3 or lower, preferably 1.9 g / cm3 or lower, more preferably 1.8 g / cm3 or lower, even more preferably 1.7 g / cm3 or lower. When the density thereof exceeds the upper limit of that range, there are cases where the active-material particles are broken and this increases the initial irreversible capacity and reduces the infiltration of a nonaqueous electrolyte into around the current collector / active material interface. As a result, high-current-density charge / discharge characteristics may decrease. When the density thereof is lower than the lower limit, there are cases where electrical conductivity among the active-material particles decreases and this increases battery resistance, resulting in a reduced capacity per unit volume.(17) Binder

[0348] The binder for binding the active material is not particularly limited as long as it is stable to the nonaqueous electrolyte and the solvent to be used for electrode production. Examples thereof include resinous polymers such as polyethylene, polypropylene, poly(ethylene terephthalate), poly(methyl methacrylate), aromatic polyamides, cellulose, and nitrocellulose; rubbery polymers such as SBR (styrene / butadiene rubbers), isoprene rubbers, butadiene rubbers, fluororubbers, NBR (acrylonitrile / butadiene rubbers), and ethylene / propylene rubbers; styrene / butadiene / styrene block copolymers or products of hydrogenation thereof; thermoplastic elastomeric polymers such as EPDM (ethylene / propylene / diene terpolymers), styrene / ethylene / butadiene / styrene copolymers, and styrene / isoprene / styrene block copolymers or products of hydrogenation thereof; flexible resinous polymers such as syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene / vinyl acetate copolymers, and propylene / α-olefin copolymers; fluorochemical polymers such as poly(vinylidene fluoride), polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and polytetrafluoroethylene / ethylene copolymers; and polymer compositions having the property of conducting alkali metal ions (especially lithium ions). One of these may be used alone, or any desired combination of two or more thereof in any desired proportion may be used.

[0349] The kind of the solvent to be used for forming a slurry is not particularly limited as long as it is a solvent in which the active material and binder and the thickener and conductive material which are optionally used according to need can be dissolved or dispersed. Either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water and alcohols. Examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, propylene oxide, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphoramide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, and hexane. Especially when an aqueous solvent is used, it is preferred to add a dispersant or the like in combination with the thickener described above and prepare a slurry using a latex of, e.g., SBR. One of such ingredients may be used alone, or any desired combination of two or more thereof in any desired proportion may be used.

[0350] The proportion of the binder to the active material is preferably 0.1% by mass or higher, especially preferably 0.5% by mass or higher, more preferably 0.6% by mass or higher. The upper limit thereof is generally 20% by mass or lower, preferably 15% by mass or lower, more preferably 10% by mass or lower, even more preferably 8% by mass or lower. In case where the binder amount exceeds the upper limit of that range, the proportion of the binder which does not contribute to battery capacity increases and this may lead to a decrease in battery capacity. When the binder amount is small than the lower limit, there are cases where the negative electrode has a reduced strength. Especially when the binder comprises a rubbery polymer represented by SBR as the main component, the proportion of this binder to the active material is generally 0.1% by mass or higher, preferably 0.5% by mass or higher, more preferably 0.6% by mass or higher. The upper limit thereof is generally 5% by mass or lower, preferably 3% by mass or lower, more preferably 2% by mass or lower. In the case where the binder comprises a fluorochemical polymer represented by poly(vinylidene fluoride) as the main component, the proportion of this binder to the active material is generally 1% by mass or higher, preferably 2% by mass or higher, more preferably 3% by mass or higher. The upper limit thereof is generally 15% by mass or lower, preferably 10% by mass or lower, more preferably 8% by mass or lower.

[0351] A thickener is used generally for the purpose of regulating the slurry viscosity. The thickener is not particularly limited. Examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, poly(vinyl alcohol), oxidized starch, phosphorylated starch, casein, and salts of these. One of these thickeners may be used alone, or any desired combination of two or more thereof in any desired proportion may be used. In the case where such a thickener is further added, the proportion of the thickener to the active material is generally 0.1% by mass or higher, preferably 0.5% by mass or higher, more preferably 0.6% by mass or higher. The upper limit thereof is generally 5% by mass or lower, preferably 3% by mass or lower, more preferably 2% by mass or lower. When the proportion thereof is lower than the lower limit of that range, there are cases where applicability decreases considerably.

[0352] Proportions thereof exceeding the upper limit result in a reduced proportion of the active material in the negative-electrode active-material layer, and this may pose a problem that battery capacity decreases and a problem that resistance among the particles of the negative-electrode active material increases.(18) Orientation Ratio in Electrode Plate

[0353] The orientation ratio in the electrode plate is preferably 0.001 or higher, more preferably 0.005 or higher, especially preferably 0.01 or higher. The upper limit thereof is up to 0.67, which is a theoretical value. When the orientation ratio therein is lower than the lower limit of that range, there are cases where high-density charge / discharge characteristics decrease.

[0354] An examination for determining the orientation ratio in the electrode plate is as follows. The negative electrode which has been pressed to a target density is examined by X-ray diffractometry to determine the orientation ratio of the active material in this electrode. Although specific techniques therefor are not particularly limited, a standard method is as follows. The peaks attributable to the (110) diffraction and (004) diffraction of the carbon obtained by X-ray diffractometry are subjected to peak separation by fitting with asymmetric Pearson VII as a profile function. Thus, the integrated intensities of the (110) diffraction and (004) diffraction peaks are calculated. From the integrated intensities obtained, the ratio represented by (integrated intensity of (110) diffraction) / (integrated intensity of (004) diffraction) is calculated. The active-material orientation ratio for the electrode thus calculated is defined as the orientation ratio in the electrode plate.

[0355] Conditions for this X-ray diffractometry are as follows. Incidentally, “2θ” represents diffraction angle.

[0356] Target: Cu(Kα line) graphite monochromator

[0357] Slit:

[0358] Divergence slit=1 degree

[0359] Receiving slit=0.1 mm

[0360] Scattering slit=1 degree

[0361] Examination range and step angle / measuring time:

[0362] (110) plane: 76.5°≤2θ≤78.5° .01° / 3 sec

[0363] (004) plane: 53.5°≤2θ≤56.0° .01° / 3 sec

[0364] Sample preparation:

[0365] The electrode is fixed to a glass plate with a double-faced pressure-sensitive adhesive tape having a thickness of 0.1 mm.(19) Impedance

[0366] At the time when the battery in a discharged state has been charged to 60% of its nominal capacity, the resistance of the negative electrode is preferably 100 Ω or lower, more preferably 50 Ω or lower, especially preferably 20 Ω or lower and / or the double-layer capacity of the negative electrode is preferably 1×10−6 F or higher, more preferably 1×10−5 F, especially preferably 1×10−4 F. This range is preferred because satisfactory output characteristics are obtained.

[0367] The resistance and double-layer capacity of the negative electrode are determined by the following procedure. The lithium secondary battery to be examined is one which, when charged at a current at which the nominal capacity can be charged over 5 hours, subsequently kept for 20 minutes in the state of being neither charged nor discharged, and then discharged at a current at which the nominal capacity can be discharged over 1 hour, has a capacity of at least 80% of the nominal capacity. This lithium secondary battery in a discharged state is charged to 60% of the nominal capacity at a current at which the nominal capacity can be charged over 5 hours. Immediately thereafter, the lithium secondary battery is transferred to a gloved box filled with an argon atmosphere. In this gloved box, the lithium secondary battery is rapidly disassembled and taken out while preventing the negative electrode from suffering discharge or short-circuiting. In the case where this negative electrode is a double-sided electrode, the electrode active material on one side is stripped off without marring the electrode active material on the other side. Two disks of 12.5 mmφ are punched out of this negative electrode, and are superposed through a separator so that the active-material sides rightly face each other. Sixty microliters of the nonaqueous electrolyte which has been used in the battery is dropped onto the separator and between the two negative electrodes, and the negative electrodes and separator are brought into tight contact with each other. While keeping this assemblage isolated from the surrounding air, the current collectors of the respective negative electrodes are electrically connected to each other and the assemblage is examined by the alternating-current impedance method. In the examination, the complex impedance is measured at a temperature of 25° C. in a frequency region of 10-2-105 Hz to determine a Cole-Cole plot. The circular arc for the negative-electrode resistance component in the plot is approximated to a semicircle to determine the surface resistance (R) and the double-layer capacity (Cdl)(20) Area and Thickness of Negative-Electrode Plate

[0368] The area of the negative-electrode plate is not particularly limited. However, the negative-electrode plate is designed to be slightly larger than the positive-electrode plate facing it so that the positive-electrode plate does not protrude outward from the negative-electrode plate. From the standpoints of cycle life in repetitions of charge / discharge and inhibition of deterioration caused by high-temperature storage, it is preferred to regulate the area of the negative electrode so as to be as close as possible to the area of the positive electrode. This is because such negative-electrode area heightens the proportion of electrodes which function more evenly and effectively and thereby improves properties. Especially when the battery is to be used at a high current, this electrode area design is important.

[0369] The thickness of the negative-electrode plate is designed so as to be suitable for the positive-electrode plate to be used, and is not particularly limited. However, the thickness of the mix layer, excluding the thickness of the core metal foil, is generally 15 μm or larger, preferably 20 μm or larger, more preferably 30 μm or larger. The upper limit thereof is generally 150 μm or smaller, preferably 120 μm or smaller, more preferably 100 μm or smaller.<Negative Electrode [1]>

[0370] Negative electrode [1], i.e., “a negative electrode containing as a negative-electrode active material two or more carbonaceous substances differing in crystallinity”, for use in a lithium secondary battery of the invention will be explained below.[Negative Electrode [1]: Negative-Electrode Active Material]

[0371] The negative-electrode active material for use in negative electrode [1] is described below.[[Constitution]]

[0372] The negative-electrode active material for use in negative electrode [1] in a lithium secondary battery of the invention is characterized by comprising two or more carbonaceous substances differing in crystallinity. The term “comprising two or more carbonaceous substances differing in crystallinity” herein means that carbonaceous substances differing in crystallinity coexist in the negative electrode. The state in which these carbonaceous substances coexist is not limited. Namely, particles of one carbonaceous substance and particles of another may be present as a mixture of these, or the carbonaceous substances may be contained in one secondary particle. Furthermore, a mixture of both is possible. The negative-electrode active material preferably is one which comprises a composite carbonaceous material comprising two or more carbonaceous substances differing in crystallinity. Also preferred is one which comprises the composite carbonaceous material and, as a minor ingredient, one or more carbonaceous substances (carbonaceous materials) differing in property from the carbonaceous substances constituting the composite carbonaceous material.

[0373] The term “contained in one secondary particle” means, for example, the state in which the carbonaceous substances differing in crystallinity are restrained by bonding, the state in which the substances are physically restrained, or the state in which the substances retain a shape by means of electrostatic restraint. The term “physically restrained” herein means a state such as that in which one of the carbonaceous substances differing in crystallinity is surrounded by the other or the state in which one is caught by the other. The term “electrostatic restraint” means the state in which one of the carbonaceous substances differing in crystallinity is adherent to the other by means of electrostatic energy. Furthermore, the term “the state in which the substances are restrained by bonding” means chemical bonding by hydrogen bonds, covalent bonds, ionic bonds, etc.

[0374] Of those, the state in which the surface of a core carbonaceous substance at least partly has an interface where a coating layer differing in crystallinity is bonded to the core carbonaceous substance is advantageous because resistance in lithium movement between the carbonaceous substances differing in crystallinity is low. The coating layer is not limited as to whether it is one formed by the bonding of an externally supplied material and / or of a product of alteration thereof or one formed by the alteration of a surface-part material of a carbonaceous substance. The term “coating” herein means that the interface between the coating layer and the surface of the carbonaceous substance at least partly has chemical bonds and the coating layer is (1) in the state of covering the whole surface, (2) in the state of locally covering the carbonaceous particles, (3) in the state of selectively covering part of the surface, or (4) in the state of being present in ultrafine regions including chemical bonds. At the interface, crystallinity may change continuously or discontinuously.

[0375] It is preferred that the composite carbonaceous material should be one which includes a particulate carbonaceous substance and has an interface where the particulate carbonaceous substance is covered with and / or bonded to a carbonaceous substance differing in crystallinity from the particulate carbonaceous substance, the interface changing in crystallinity discontinuously and / or continuously. There are no limitations on which of the “particulate carbonaceous substance” and the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” has higher crystallinity. However, from the standpoint of producing the effects of the invention, it is preferred that the particulate carbonaceous substance should have higher crystallinity.

[0376] A difference in crystallinity herein is judged based on a difference in interplanar spacing (d002) for the (002) planes, difference in Lc, or difference in La, each determined by wide-angle X-ray diffractometry. From the standpoint of producing the effects of the invention, the difference in crystallinity is preferably 0.0002 nm or larger in terms of (d002), or 1 nm or larger in terms of La, or 1 nm or larger in terms of Lc. The difference in (d002), among those ranges, is preferably 0.0005 nm or larger, more preferably 0.001 nm or larger, even more preferably 0.003 nm or larger. The upper limit thereof is generally 0.03 nm or smaller, preferably 0.02 nm or smaller. When the difference therein is smaller than the lower limit of that range, there are cases where the effect of the difference in crystallinity becomes low. On the other hand, in case where the difference therein exceeds the upper limit of that range, the part having lower crystallinity tends to be too low in crystallinity and there is a possibility that the reduced crystallinity might increase irreversible capacity. The difference in La or Lc, among those ranges, is preferably 2 nm or larger, more preferably 5 nm or larger, even more preferably 10 nm or larger. In graphites, values larger than 100 nm cannot be specified because definition is generally impossible. When the difference in La or Lc is smaller than the lower limit of that range, there are cases where the effect of the difference in crystallinity becomes low.

[0377] As stated above, the composite carbonaceous material may comprise a particulate carbonaceous substance and a carbonaceous substance which differs in crystallinity from the particulate carbonaceous substance and which covers and / or is bonded to the particulate carbonaceous substance. In this material, either of the “particulate carbonaceous substance” and the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” may be a graphitic carbonaceous substance and the other may be a lowly crystalline carbonaceous substance. It is preferred that the “particulate carbonaceous substance” should be a graphitic carbonaceous substance and the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” be a lowly crystalline carbonaceous substance.[[[Particulate Carbonaceous Substance]]]

[0378] The particulate carbonaceous substance preferably is a graphitic carbonaceous substance comprising a natural graphite and / or an artificial graphite or is one comprising at least one member selected from the group consisting of the following (a), (b), and (c), which have slightly lower crystallinity than those graphites:(a) a product of organic-substance pyrolysis selected from the group consisting of coal coke, petroleum coke, furnace black, acetylene black, and pitch-derived carbon fibers;(b) a product of the carbonization of the gas of an organic substance; and(c) a carbonaceous substance obtained by partly or wholly graphitizing (a) or (b).[[[[Graphitic Carbonaceous Substance]]]]

[0379] The particulate carbonaceous substance preferably is a graphitic carbonaceous substance comprising a natural graphite and / or an artificial graphite. The term graphitic carbonaceous substance means any of various carbonaceous substances which have such high crystallinity that the interplanar spacing (d002) for the (002) planes thereof as determined by wide-angle X-ray diffractometry is smaller than 0.340 nm.

[0380] Preferred examples of the graphitic carbonaceous substance include powders selected from natural graphites, artificial graphites, products obtained by the mechanical pulverization or re-heat treatment of these graphites, products of the re-heat treatment of an expanded graphite, and highly purified products obtained from those graphites. Preferred examples of the artificial graphites are ones obtained by graphitizing one or more organic substances selected from coal tar pitch, coal-derived heavy oil, topping residues, petroleum-derived heavy oil, aromatic hydrocarbons, nitrogen-containing cyclic compounds, sulfur-containing cyclic compounds, polyphenylenes, poly(vinyl chloride), poly(vinyl alcohol), polyacrylonitrile, poly(vinyl butyral), natural polymers, poly(phenylene sulfide), poly(phenylene oxide), furfuryl alcohol resins, phenol-formaldehyde resins, imide resins, and the like at a heat-treating temperature of generally about from 2,500° C. to 3,200° C. and powdering the graphitized substances by an appropriate pulverizer.(Properties of Graphitic Carbonaceous Substance)

[0381] It is desirable that the graphitic carbonaceous substance should satisfy any one of the following properties (1) to (11) or simultaneously satisfy two or more thereof.(1) X-Ray Parameter

[0382] The graphitic carbonaceous substance preferably has a value of d (interplanar spacing) for the lattice planes (002), as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, of 0.335 nm or larger. The lower limit of the value of d is smaller than 0.340 nm according to the definition, and is preferably 0.337 nm or smaller. When the value of dis too large, there are cases where crystallinity decreases to increase initial irreversible capacity. On the other hand, 0.335 is a theoretical value for graphites. Furthermore, the crystallite size (Lc) of this carbon material, as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is generally 30 nm or larger, preferably 50 nm or larger, more preferably 100 nm or larger. In case where the Lc thereof is smaller than the lower limit of that range, there is a possibility that crystallinity might decrease to increase initial irreversible capacity.(2) Ash Content

[0383] The ash content of the graphitic carbonaceous substance is preferably 1% by mass or lower, especially 0.5% by mass or lower, in particular 0.1% by mass or lower, based on the whole graphitic carbonaceous substance. The lower limit of the ash content thereof is preferably at least 1 ppm by mass of the whole carbonaceous substance. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the electrolyte during charge / discharge becomes not negligible. When the ash content thereof is lower than the lower limit of that range, there are cases where the production of this carbonaceous material necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.(3) Volume-Average Particle Diameter

[0384] With respect to the volume-average particle diameter of the graphitic carbonaceous substance, the volume-average particle diameter (median diameter) thereof as determined by the laser diffraction / scattering method is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger, even more preferably 7 μm or larger. The upper limit thereof is generally 100 μm or smaller, preferably 50 μm or smaller, more preferably 40 μm or smaller, even more preferably 30 μm or smaller, especially preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where irreversible capacity increases, leading to a loss in initial battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such a carbonaceous substance is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode plate is produced.

[0385] Volume-average particle diameter in the invention is defined as the median diameter determined by dispersing the carbon powder in a 0.2% by mass aqueous solution (about 1 mL) of poly (oxyethylene (20)) sorbitan monolaurate as a surfactant and examining the dispersion with a laser diffraction type particle size distribution analyzer (e.g., LA-700, manufactured by HORIBA, Ltd.).(4) Raman R Value, Raman Half-Value Width

[0386] The R value of the graphitic carbonaceous substance as determined by the argon ion laser Raman spectroscopy is generally 0.01 or higher, preferably 0.03 or higher, more preferably 0.10 or higher. The upper limit thereof is generally 0.60 or lower, preferably 0.40 or lower. When the R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. On the other hand, when the R value thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0387] The Raman half-value width around 1,580 cm−1 of the graphitic carbonaceous substance is not particularly limited. However, the half-value width thereof is generally 10 cm−1 or larger, preferably 15 cm−1 or larger. The upper limit thereof is generally 60 cm−1 or smaller, preferably 45 cm−1 or smaller, more preferably 40 cm−1 or smaller. When the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. On the other hand, when the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0388] The examination for a Raman spectrum is made with a Raman spectrometer (e.g., a Raman spectrometer manufactured by Japan Spectroscopic Co., Ltd.). In the examination, a sample is charged into a measuring cell by causing the sample to fall naturally into the cell and the surface of the sample in the cell is irradiated with argon ion laser light while rotating the cell in a plane perpendicular to the laser light. The Raman spectrum obtained is examined for the intensity IA of a peak PA around 1,580 cm−1 and the intensity IB of a peak PB around 1,360 cm−1. The ratio between these intensities R (R=IB / IA) is calculated and this ratio is defined as the Raman R value of the carbon material. Furthermore, the half-value width of the peak PA around 1,580 cm−1 in the Raman spectrum obtained is measured, and this value is defined as the Raman half-value width of the carbon material.

[0389] Conditions for this Raman spectroscopy are as follows.

[0390] Wavelength of argon ion laser: 514.5 nm

[0391] Laser power on sample: 15-25 mW

[0392] Resolution: 10-20 cm−1

[0393] Examination range: 1,100-1,730 cm−1

[0394] Analysis for R value and half-value width: background processing

[0395] Smoothing: simple average; convolution, 5 points(5) BET Specific Surface Area

[0396] The specific surface area of the graphitic carbonaceous substance, as determined by the BET method, is generally 0.1 m2 / g or larger, preferably 0.7 m2 / g or larger, more preferably 1 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is generally 100 m2 / g or smaller, preferably 25 m2 / g or smaller, more preferably 15 m2 / g or smaller, even more preferably 10 m2 / g or smaller. When the specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this graphitic carbonaceous substance as a negative-electrode material is apt to result in poor lithium acceptance during charge and in lithium deposition on the electrode surface. On the other hand, when the specific surface area thereof exceeds the upper limit of that range, there are cases where use of this graphitic carbonaceous substance as a negative-electrode material is apt to result in enhanced reactivity with the electrolyte and enhanced gas evolution and a preferred battery is difficult to obtain.

[0397] BET specific surface area is defined as a value measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by preliminarily drying a sample at 350° C. for 15 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.(6) Pore Distribution

[0398] In the graphitic carbonaceous substance, the amount of interstices corresponding to a pore diameter of from 0.01 μm to 1 μm which include pores in the particles and particle surface irregularities formed by steps, as determined by mercury porosimetry (mercury intrusion method), is generally 0.01 mL / g or larger, preferably 0.05 mL / g or larger, more preferably 0.1 mL / g or larger. The upper limit thereof is generally 0.6 mL / g or smaller, preferably 0.4 mL / g or smaller, more preferably 0.3 mL / g or smaller. When the amount of interstices therein is larger than the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. On the other hand, when the amount thereof is smaller than the lower limit of that range, there are cases where high-current-density charge / discharge characteristics decrease and the effect of diminishing electrode expansion / contraction during charge / discharge is not obtained.

[0399] The total pore volume is preferably 0.1 mL / g or larger, more preferably 0.25 mL / g or larger. The upper limit thereof is generally 10 mL / g or smaller, preferably 5 mL / g or smaller, more preferably 2 mL / g or smaller. When the total pore volume exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the total pore volume is smaller than the lower limit, there are cases where the dispersing effect of a thickener or binder in electrode plate formation is not obtained.

[0400] The average pore diameter of the graphitic carbonaceous substance is preferably 0.05 μm or larger, more preferably 0.1 μm or larger, even more preferably 0.5 μm or larger. The upper limit thereof is generally 50 μm or smaller, preferably 20 μm or smaller, more preferably 10 μm or smaller. When the average pore diameter thereof exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary. On the other hand, when the average pore diameter thereof is smaller than the lower limit of that range, there are cases where high-current-density charge / discharge characteristics decrease.

[0401] As an apparatus for the mercury porosimetry is used a mercury porosimeter (Autopore 9520, manufactured by Micromeritics Corp.). About 0.2 g of a sample (negative-electrode material) is weighed out and placed in a powder cell and this cell is closed. The sample in the cell is pretreated by degassing it at room temperature under vacuum (50 μmHg or lower) for 10 minutes. Subsequently, the pressure in the cell is reduced to 4 psia (about 28 kPa) and mercury is introduced thereinto. The internal pressure is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia (about 280 MPa) and then lowered to 25 psia (about 170 kPa). The number of steps in the pressure elevation is 80 or larger. In each step, the amount of mercury intruded is measured after an equilibrium time of 10 seconds. A pore distribution is calculated from the mercury intrusion curve thus obtained, using the Washburn equation. For the calculation, the surface tension (γ) and contact angle (ψ) of mercury are taken as 485 dyne / cm and 140°, respectively. The average pore diameter used is the pore diameter corresponding to a cumulative pore volume of 50%.(7) Roundness

[0402] With respect to the degree of sphericity of the graphitic carbonaceous substance, the roundness of the particles having a particle diameter in the range of 3-40 μm is preferably 0.1 or higher, especially preferably 0.5 or higher, more preferably 0.8 or higher, even more preferably 0.85 or higher, most preferably 0.9 or higher. High roundness values are preferred because they are effective in improving high-current-density charge / discharge characteristics. Roundness is defined by the following equation. When a particle has a roundness of 1, this particle theoretically is a true sphere.Roundness=(length of periphery of equivalent circle having the same area as projected particle shape) / (actual length of periphery of projected particle shape)

[0403] The value of roundness is determined, for example, with a flow type particle image analyzer (e.g., FPIA, manufactured by Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene (20)) sorbitan monolaurate as a surfactant, propagating an ultrasonic wave of 28 kHz to the dispersion for 1 minute at an output of 60 W, and then examining particles having a particle diameter in the range of 3-40 μm with the analyzer having a detection range set at 0.6-400 μm.

[0404] Methods for improving roundness are not particularly limited. However, a graphitic carbonaceous substance in which the particles have been rounded by a rounding treatment is preferred because it gives an electrode in which the interstices among particles are uniform in shape. Examples of the rounding treatment include: a method in which a shear force or compressive force is applied to thereby mechanically make the shape of the particles close to sphere; and a method of mechanical / physical treatment in which fine particles are aggregated into particles by means of the bonding force of a binder or of the fine particles themselves.(8) True Density

[0405] The true density of the graphitic carbonaceous substance is generally 2 g / cm3 or higher, preferably 2.1 g / cm3 or higher, more preferably 2.2 g / cm3 or higher, even more preferably 2.22 g / cm3 or higher. The upper limit thereof may be 2.26 g / cm3 or lower. This upper limit is a theoretical value for graphites. When the true density of the graphitic carbonaceous substance is lower than the lower limit of that range, there are cases where this carbon has too low crystallinity, resulting in an increase in initial irreversible capacity. In the invention, true density is defined as one determined by the liquid-phase displacement method (pycnometer method) using butanol.(9) Tap Density

[0406] The tap density of the graphitic carbonaceous substance desirably is generally 0.1 g / cm3 or higher, preferably 0.5 g / cm3 or higher, more preferably 0.7 g / cm3 or higher, especially preferably 0.9 g / cm3 or higher. The upper limit thereof is preferably 2 g / cm3 or lower, more preferably 1.8 g / cm3 or lower, especially preferably 1.6 g / cm3 or lower. When the tap density thereof is lower than the lower limit of that range, there are cases where this graphitic carbonaceous substance, when used in a negative electrode, is less apt to have a high loading density and cannot give a battery having a high capacity. On the other hand, when the tap density thereof exceeds the upper limit of that range, the amount of interparticle interstices in the electrode is too small and it is difficult to secure electrical conductivity among the particles. There are hence cases where preferred battery performances are difficult to obtain.

[0407] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the cell with the sample up to the brim, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining the resultant bulk density. This bulk density is defined as the tap density.(10) Orientation Ratio (Powder)

[0408] The orientation ratio of the graphitic carbonaceous substance is generally 0.005 or higher, preferably 0.01 or higher, more preferably 0.015 or higher. The upper limit thereof is theoretically 0.67 or lower. When the orientation ratio thereof is lower than the lower limit of that range, there are cases where high-density charge / discharge characteristics decrease.

[0409] Orientation ratio is determined by X-ray diffractometry. The peaks attributable to the (110) diffraction and (004) diffraction of the carbon obtained by X-ray diffractometry are subjected to peak separation by fitting with asymmetric Pearson VII as a profile function. Thus, the integrated intensities of the (110) diffraction and (004) diffraction peaks are calculated. From the integrated intensities obtained, the ratio represented by (integrated intensity of (110) diffraction) / (integrated intensity of (004) diffraction) is calculated. This ratio is defined as the active-material orientation ratio.

[0410] Conditions for this X-ray diffractometry are as follows. Incidentally, “2θ” represents diffraction angle.

[0411] Target: Cu(Kα line) graphite monochromator

[0412] Slit:

[0413] Divergence slit=1 degree

[0414] Receiving slit=0.1 mm

[0415] Scattering slit=1 degree

[0416] Examination range and step angle / measuring time:

[0417] (110) plane: 76.5°≤2θ≤78.5° .01° / 3 sec

[0418] (004) plane: 53.5°≤2θ≤56.0° .01° / 3 sec(11) Aspect Ratio (Powder)

[0419] The aspect ratio of the graphitic carbonaceous substance is theoretically 1 or higher. The upper limit thereof is generally 10 or lower, preferably 8 or lower, more preferably 5 or lower. When the aspect ratio thereof exceeds the upper limit, there are cases where this graphitic carbonaceous substance causes streak lines in electrode plate formation and an even coating surface cannot be obtained, resulting in a decrease in high-current-density charge / discharge characteristics.

[0420] The aspect ratio of a carbon material particle is represented by A / B, wherein A is the length of the longest axis of the particle in a three-dimensional examination and B is the length of the shortest axis perpendicular to that axis. The examination of carbon particles is made with a scanning electron microscope capable of enlargement examination. Fifty are arbitrarily selected from graphite particles fixed to an edge face of a metal having a thickness of 50 μm or smaller, and each particle is examined for A and B while rotating or inclining the stage to which the sample is fixed. The average of the A / B values is determined.[[[[Lowly Crystalline Carbonaceous Substance]]]]

[0421] The term lowly crystalline carbonaceous substance means a carbonaceous substance which has such low crystallinity that the interplanar spacing (d002) for the (002) planes as determined by wide-angle X-ray diffractometry is 0.340 nm or larger.(Composition of Lowly Crystalline Carbonaceous Substance)

[0422] The “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” preferably is a carbonaceous substance having lower crystallinity than the particulate carbonaceous substance. It preferably is a product of the carbonization of the following (d) or (e): (d) an organic substance capable of carbonization which is selected from the group consisting of coal-derived heavy oil, straight-run heavy oil, petroleum-derived cracked heavy oil, aromatic hydrocarbons, nitrogen-ring compounds, sulfur-ring compounds, polyphenylenes, organic synthetic polymers, natural polymers, thermoplastic resins, and thermosetting resins; and (e) a solution obtained by dissolving any of these organic substances capable of carbonization in a low-molecular organic solvent.

[0423] The coal-derived heavy oil preferably is any of coal tar pitches ranging from soft pitch to hard pitch, dry distillation / liquefaction oil, and the like. The straight-run heavy oil preferably is any of topping residues, vacuum distillation residues, and the like. The petroleum-derived cracked heavy oil preferably is, for example, ethylene tar as a by-product of the thermal cracking of crude oil, naphtha, etc. The aromatic hydrocarbons preferably are acenaphthylene, decacyclene, anthracene, phenanthrene, and the like. The nitrogen-ring compounds preferably are phenazine, acridine, and the like. The sulfur-ring compounds preferably are thiophene, bithiophene, and the like. The polyphenylenes preferably are biphenyl, terphenyl, and the like. The organic synthetic polymers preferably are poly(vinyl chloride), poly(vinyl alcohol), poly (vinyl butyral), substances obtained by insolubilizing these compounds, polyacrylonitrile, polypyrrole, polythiophene, polystyrene, and the like. The natural polymers preferably are, for example, polysaccharides such as cellulose, lignin, mannan, poly(galacturonic acid), chitosan, saccharose, and the like. The thermoplastic resins preferably are poly(phenylene sulfide), poly(phenylene oxide), and the like. The thermosetting resins preferably are furfuryl alcohol resins, phenol-formaldehyde resins, imide resins, and the like.

[0424] Although the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” preferably is a product of the carbonization of the “organic substance capable of carbonization” described above, it also preferably is a product of the carbonization of a solution obtained by dissolving the “organic substance capable of carbonization” in a low-molecular organic solvent such as benzene, toluene, xylene, quinoline, or n-hexane. It is furthermore preferred that the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” should be a product of the carbonization of coal coke or petroleum coke.

[0425] It is especially preferred that the (d) or (e) should be liquid. Namely, to cause carbonization to proceed in a liquid phase is preferred from the standpoint of forming an interface between the lowly crystalline carbonaceous substance and a graphitic carbonaceous substance part.(Properties of Lowly Crystalline Carbonaceous Substance)

[0426] It is desirable that the lowly crystalline carbonaceous substance should satisfy any one of the following properties (1) to (5) or simultaneously satisfy two or more thereof. Furthermore, one lowly crystalline carbonaceous substance having such properties may be used alone, or any two or more such carbonaceous substances may be used in combination in any desired proportion.(1) X-Ray Parameter

[0427] With respect to the lowly crystalline carbonaceous substance part, the value of d (interplanar spacing) for the lattice planes (002), as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is 0.340 nm or larger according to the definition. However, the value of d thereof is preferably 0.340 nm or larger, especially preferably 0.341 nm or larger. The upper limit thereof is preferably 0.380 nm or smaller, especially preferably 0.355 nm or smaller, more preferably 0.350 nm or smaller. When the value of d is too large, there are cases where the surface of this carbonaceous substance has considerably low crystallinity to increase irreversible capacity. When the value of d is too small, there are cases where the effect of improving suitability for charge which is obtained by disposing a lowly crystalline carbonaceous substance on the surface is lessened to reduce the effects of the invention. Furthermore, the crystallite size (Lc) of this carbon material, as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is generally 1 nm or larger, preferably 1.5 nm or larger. In case where the Lc thereof is smaller than the lower limit of that range, there are cases where crystallinity decreases to increase initial irreversible capacity.(2) Ash Content

[0428] The ash content of the lowly crystalline carbonaceous substance part may be 1% by mass or lower and is especially 0.5% by mass or lower, in particular 0.1% by mass or lower, based on the whole composite carbonaceous material. The lower limit of the ash content thereof is preferably at least 1 ppm by mass of the whole carbonaceous material. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the electrolyte during charge / discharge becomes not negligible. When the ash content thereof is lower than the lower limit of that range, there are cases where the production of this carbonaceous substance necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.(4) Raman R Value, Raman Half-Value Width

[0429] The R value of the lowly crystalline carbonaceous substance part as determined by the argon ion laser Raman spectroscopy is generally 0.5 or higher, preferably 0.7 or higher, more preferably 0.9 or higher. The upper limit thereof is generally 1.5 or lower, preferably 1.2 or lower. When the R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a composite carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the R value thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0430] The Raman half-value width around 1,580 cm−1 of the lowly crystalline carbonaceous substance part is not particularly limited. However, the half-value width thereof is generally 40 cm−1 or larger, preferably 50 cm−1 or larger. The upper limit thereof is generally 100 cm−1 or smaller, preferably 90 cm−1 or smaller, more preferably 80 cm−1 or smaller. When the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a composite carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.(4) True Density

[0431] The true density of the lowly crystalline carbonaceous substance part is generally 1.4 g / cm3 or higher, preferably 1.5 g / cm3 or higher, more preferably 1.6 g / cm3 or higher, even more preferably 1.7 g / cm3 or higher. The upper limit thereof is generally 2.1 g / cm3 or lower, preferably 2 g / cm3 or lower. In case where the true density thereof exceeds the upper limit of that range, there is a possibility that suitability for charge might be impaired. When the true density thereof is lower than the lower limit of that range, there are cases where this carbon has too low crystallinity, resulting in an increase in initial irreversible capacity.(5) Orientation Ratio

[0432] The orientation ratio of the lowly crystalline carbonaceous substance part is generally 0.005 or higher, preferably 0.01 or higher, more preferably 0.015 or higher. The upper limit thereof is theoretically 0.67 or lower. Orientation ratios thereof lower than the lower limit of that range are undesirable because high-density charge / discharge characteristics decrease. Orientation ratio is determined by X-ray diffractometry. The peaks attributable to the (110) diffraction and (004) diffraction of the carbon obtained by X-ray diffractometry are subjected to peak separation by fitting with asymmetric PearsonVII as a profile function. Thus, the integrated intensities of the (110) diffraction and (004) diffraction peaks are calculated. From the integrated intensities obtained, the ratio represented by (integrated intensity of (110) diffraction) / (integrated intensity of (004) diffraction) is calculated. This ratio is defined as the electrode active-material orientation ratio.[[[Composite Carbonaceous Material]]]

[0433] The composite carbonaceous material for use in negative electrode [1] in a lithium secondary battery of the invention preferably comprises a “particulate carbonaceous substance” and a “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” as stated above. In this case, either of these substances may be a graphitic carbonaceous substance and the other may be a lowly crystalline carbonaceous substance. Preferably, the “particulate carbonaceous substance” is a graphitic carbonaceous substance and the “carbonaceous substance differing in crystallinity from the particulate carbonaceous substance” is a lowly crystalline carbonaceous substance.

[0434] In the composite carbonaceous material, the proportion of the graphitic carbonaceous substance to the lowly crystalline carbonaceous substance by mass is preferably 50 / 50 or higher, more preferably 80 / 20 or higher, especially preferably 90 / 10 or higher, and is preferably 99.9 / 0.1 or lower, more preferably 99 / 1 or lower, especially preferably 98 / 2 or lower. When the proportion thereof exceeds the upper limit of that range, there are cases where the possession of the two kinds of crystalline carbonaceous substances does not produce its effect. In case where the proportion thereof is lower than the lower limit of that range, initial irreversible capacity tends to increase and this may pose a problem concerning battery design.

[0435] It is preferred that the graphitic carbonaceous substance should account for 50% by mass or more of the whole composite carbonaceous material.(Properties of Composite Carbonaceous Material)

[0436] It is desirable that the composite carbonaceous material should satisfy any one of the following properties (1) to (11) or simultaneously satisfy two or more thereof. Furthermore, one composite carbonaceous material having such properties may be used alone, or any two or more such composite carbonaceous materials may be used in combination in any desired proportion.(1) X-Ray Parameter

[0437] The composite carbonaceous material preferably has a value of d (interplanar spacing) for the lattice planes (002), as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, of 0.335 nm or larger. The value of d desirably is generally 0.350 nm or smaller, preferably 0.345 nm or smaller, more preferably 0.340 nm or smaller. Furthermore, the crystallite size (Lc) of this carbon material, as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is generally 1.5 nm or larger, preferably 3.0 nm or larger. In case where the Lc thereof is smaller than the lower limit of that range, there is a possibility that crystallinity might decrease to increase initial irreversible capacity.(2) Ash Content

[0438] The ash content of the composite carbonaceous material is generally 1% by mass or lower, especially 0.5% by mass or lower, in particular 0.1% by mass or lower, based on the whole composite carbonaceous material. The lower limit of the ash content thereof is preferably at least 1 ppm by mass of the whole composite carbonaceous material. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the electrolyte during charge / discharge becomes not negligible. When the ash content thereof is lower than the lower limit of that range, there are cases where the production of this composite carbonaceous material necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.(3) Volume-Average Particle Diameter

[0439] With respect to the volume-average particle diameter of the composite carbonaceous material, the volume-average particle diameter (median diameter) thereof as determined by the laser diffraction / scattering method is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger, even more preferably 7 μm or larger. The upper limit thereof is generally 100 μm or smaller, preferably 50 μm or smaller, more preferably 40 μm or smaller, even more preferably 30 μm or smaller, especially preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where irreversible capacity increases, leading to a loss in initial battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such a composite carbonaceous material is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode plate is produced.(4) Raman R Value, Raman Half-Value Width

[0440] The R value of the composite carbonaceous material as determined by the argon ion laser Raman spectroscopy is generally 0.03 or higher, preferably 0.10 or higher, more preferably 0.15 or higher. The upper limit thereof may be 0.60 or lower and is preferably 0.50 or lower. When the R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a composite carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the R value thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.

[0441] The Raman half-value width around 1,580 cm−1 of the composite carbonaceous material is not particularly limited. However, the half-value width thereof is generally 15 cm−1 or larger, preferably 20 cm−1 or larger. The upper limit thereof is generally 70 cm−1 or smaller, preferably 60 cm−1 or smaller, more preferably 50 cm−1 or smaller. When the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. In addition, when a coating fluid containing such a composite carbonaceous material is applied to a current collector and the resultant coating is pressed to heighten the density of the negative electrode, then the crystals are apt to orient in directions parallel to the electrode plate and this may lead to a decrease in load characteristics. On the other hand, when the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and enhanced reactivity with the electrolyte and this may lead to a decrease in efficiency and enhanced gas evolution.(5) BET Specific Surface Area

[0442] The specific surface area of the composite carbonaceous material, as determined by the BET method, is generally 0.1 m2 / g or larger, preferably 0.7 m2 / g or larger, more preferably 1 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is generally 100 m2 / g or smaller, preferably 25 m2 / g or smaller, more preferably 15 m2 / g or smaller, even more preferably 10 m2 / g or smaller. When the specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this composite carbonaceous material as a negative-electrode material is apt to result in poor lithium acceptance during charge and in lithium deposition on the electrode surface. On the other hand, when the specific surface area thereof exceeds the upper limit of that range, there are cases where use of this composite carbonaceous material as a negative-electrode material is apt to result in enhanced reactivity with the electrolyte and enhanced gas evolution and a preferred battery is difficult to obtain.(6) Pore Distribution

[0443] In the composite carbonaceous material, the amount of interstices corresponding to a pore diameter of from 0.01 μm to 1 μm which include pores in the particles and particle surface irregularities formed by steps, as determined by mercury porosimetry (mercury intrusion method), is generally 0.01 mL / g or larger, preferably 0.05 mL / g or larger, more preferably 0.1 mL / g or larger. The upper limit thereof is generally 0.6 mL / g or smaller, preferably 0.4 mL / g or smaller, more preferably 0.3 mL / g or smaller. When the amount of interstices therein is larger than the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the amount thereof is smaller than the lower limit, there are cases where high-current-density charge / discharge characteristics decrease and the effect of diminishing electrode expansion / contraction during charge / discharge is not obtained.

[0444] The total pore volume is preferably 0.1 mL / g or larger, more preferably 0.25 mL / g or larger. The upper limit thereof is generally 10 mL / g or smaller, preferably 5 mL / g or smaller, more preferably 2 mL / g or smaller. When the total pore volume exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the total pore volume is smaller than the lower limit, there are cases where the dispersing effect of a thickener or binder in electrode plate formation is not obtained.

[0445] The average pore diameter of the composite carbonaceous material is preferably 0.05 μm or larger, more preferably 0.1 μm or larger, even more preferably 0.5 μm or larger. The upper limit thereof is generally 80 μm or smaller, preferably 50 μm or smaller, more preferably 20 μm or smaller. When the average pore diameter thereof exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary. When the average pore diameter thereof is smaller than the lower limit, there are cases where high-current-density charge / discharge characteristics decrease.(7) Roundness

[0446] With respect to the degree of sphericity of the composite carbonaceous material, the roundness of the particles having a particle diameter in the range of 3-40 μm is preferably 0.85 or higher, more preferably 0.87 or higher, even more preferably 0.9 or higher. High roundness values are preferred because they are effective in improving high-current-density charge / discharge characteristics.(8) True Density

[0447] The true density of the composite carbonaceous material is generally 1.9 g / cm3 or higher, preferably 2 g / cm3 or higher, more preferably 2.1 g / cm3 or higher, even more preferably 2.2 g / cm3 or higher. The upper limit thereof may be 2.26 g / cm3 or lower. This upper limit is a theoretical value for graphites. When the true density of the composite carbonaceous material is lower than the lower limit of that range, there are cases where this carbon has too low crystallinity, resulting in an increase in initial irreversible capacity.(9) Tap Density

[0448] The tap density of the composite carbonaceous material desirably is generally 0.1 g / cm3 or higher, preferably 0.5 g / cm3 or higher, more preferably 0.7 g / cm3 or higher, especially preferably 1 g / cm3 or higher. The tap density thereof is preferably 2 g / cm3 or lower, more preferably 1.8 g / cm3 or lower, especially preferably 1.6 g / cm3 or lower. When the tap density thereof is lower than the lower limit of that range, there are cases where this composite carbonaceous material, when used in a negative electrode, is less apt to have a high loading density and cannot give a battery having a high capacity. On the other hand, when the tap density thereof exceeds the upper limit of that range, the amount of interparticle interstices in the electrode is too small and it is difficult to secure electrical conductivity among the particles. There are hence cases where preferred battery performances are difficult to obtain. Tap density is determined by the same method as described above and is defined as described therein.(10) Orientation Ratio (Powder)

[0449] The orientation ratio of the composite carbonaceous material is generally 0.005 or higher, preferably 0.01 or higher, more preferably 0.015 or higher. The upper limit thereof is theoretically 0.67 or lower. When the orientation ratio thereof is lower than the lower limit of that range, there are cases where high-density charge / discharge characteristics decrease.(11) Aspect Ratio (Powder)

[0450] The aspect ratio of the composite carbonaceous material is theoretically 1 or higher. The upper limit thereof may be 10 or lower and is preferably 8 or lower, more preferably 5 or lower. When the aspect ratio thereof exceeds the upper limit, there are cases where this composite carbonaceous material causes streak lines in electrode plate formation and an even coating surface cannot be obtained, resulting in a decrease in high-current-density charge / discharge characteristics.(Processes for Producing Composite Carbonaceous Material)

[0451] Processes for producing the composite carbonaceous material are not particularly limited. Examples thereof include the following methods.

[0452] Methods usable for combining a graphitic carbonaceous substance with a lowly crystalline carbonaceous substance include: a method in which a carbon precursor for obtaining a lowly crystalline carbonaceous substance is used as it is and a mixture of the carbon precursor and a powder of a graphitic carbonaceous substance is heat-treated to obtain the composite powder; a method which comprises partly carbonizing the carbon precursor to produce a powder of a lowly crystalline carbonaceous substance beforehand, mixing this powder with a powder of a graphitic carbonaceous substance, and heat-treating the mixture to combine the two substances; and a method which comprises producing the powder of a lowly crystalline carbonaceous substance beforehand, mixing a powder of a graphitic carbonaceous substance, the powder of a lowly crystalline carbonaceous substance, and a carbon precursor, and heat-treating the mixture to combine these substances. In the latter two methods, in which a powder of a lowly crystalline carbonaceous substance is prepared beforehand, it is preferred that the particles of the lowly crystalline carbonaceous substance to be used should have an average particle diameter not larger than one-tenth the average particle diameter of the particles of the graphitic carbonaceous substance. Also usable is a method in which the lowly crystalline carbonaceous substance produced beforehand is mixed with a graphitic carbonaceous substance while applying mechanical energy thereto by, e.g., pulverization to thereby impart thereto a structure in which one is surrounded by the other or a structure in which one is electrostatically adherent to the other.

[0453] In a preferred process, a mixture of particles of a graphitic carbonaceous substance and a carbon precursor is heated to obtain an intermediate, or a mixture of particles of a graphitic carbonaceous substance and particles of a lowly crystalline carbonaceous substance is mixed with a carbon precursor and this mixture is heated to obtain an intermediate. Thereafter, the intermediate is carbonized / heat-treated and pulverized to finally obtain a composite carbonaceous material comprising the particles of a graphitic carbonaceous substance and a lowly crystalline carbonaceous substance combined therewith.

[0454] This process for obtaining a composite carbonaceous material is divided into the following four steps.

[0455] First step: Either particles of a graphitic carbonaceous substance or a mixture of particles of a graphitic carbonaceous substance and particles of a lowly crystalline carbonaceous substance is mixed with a carbon precursor for particles of a lowly crystalline carbonaceous substance and optionally with a solvent by means of any of various commercial mixers, kneaders, etc. to obtain a mixture.

[0456] Second step: The mixture is heated to obtain an intermediate from which the solvent and any volatile ingredient generating from the carbon precursor have been removed. According to need, this heating may be conducted while stirring the mixture. Even when a volatile ingredient remains, this poses no problem because it will be removed in the subsequent step, i.e., the third step.

[0457] Third step: The mixture or intermediate is heated to a temperature of from 400° C. to 3,200° C. in an inert gas atmosphere such as nitrogen gas, carbon dioxide gas, or argon gas to obtain a graphite / lowly crystalline carbonaceous substance composite.

[0458] Fourth step: The composite is subjected to powder processing such as pulverization, disaggregation, classification, etc. according to need.

[0459] Of those steps, the second step and the fourth step can be omitted in some cases, and the fourth step may be conducted before the third step. However, in the case where the fourth step was conducted before the third step, powder processing such as pulverization, disaggregation, classification, etc. is conducted again according to need to obtain a composite carbonaceous material.

[0460] With respect to conditions for the heat treatment in the third step, heat hysteresis temperature conditions are important. The lower limit of the temperature is generally 400° C. or higher, preferably 900° C. or higher, although it changes slightly depending on the kind of the carbon precursor and the heat hysteresis thereof. On the other hand, with respect to upper-limit temperature, the mixture or intermediate can be heated basically to a temperature at which the carbon precursor does not come to have a structural order higher than the crystal structure of the graphitic carbonaceous substance particles as cores. Consequently, the upper-limit temperature in the heat treatment is generally 3,200° C. or lower, preferably 2,000° C. or lower, more preferably 1,500° C. or lower. Heating rate, cooling rate, heat treatment period, and the like, among conditions of such heat treatment, can be determined at will according to purposes. It is also possible to conduct a heat treatment in a relatively low-temperature region and then heat the work to a given temperature. The reactor to be used in the steps may be of the batch type or continuous type, and one reactor or two or more reactors may be used.[[Minor-Material Mixing]]

[0461] Besides the composite carbonaceous material, one or more carbonaceous substances (carbonaceous materials) differing in carbonaceous-substance property from the composite carbonaceous material may be incorporated into the negative-electrode active material for use in a lithium secondary battery of the invention. Thus, battery performances can be further improved. The term “carbonaceous-substance property” herein means one or more properties selected from X-ray diffraction parameter, median diameter, aspect ratio, BET specific surface area, orientation ratio, Raman R value, tap density, true density, pore distribution, roundness, and ash content. Preferred embodiments include: one in which the volume-based particle size distribution is not symmetrical about the median diameter; one in which two or more carbon materials differing in Raman R value are contained; and one in which carbon materials differing in X-ray parameter are contained. One example of the effects thereof is that the incorporation of a carbon material, such as a graphite, e.g., a natural graphite or artificial graphite, or an amorphous carbon, e.g., a carbon black such as acetylene black or needle coke, as a sub material serves to reduce electrical resistance. One of these carbon materials may be used alone, or any desired two or more thereof may be used in combination in any desired proportion. In the case where such a carbon material is added as a sub material, the amount thereof may be 0.1% by mass or larger and is preferably 0.5% by mass or larger, more preferably 0.6% by mass or larger. The upper limit thereof may be 80% by mass or smaller and is preferably 50% by mass or smaller, more preferably 40% by mass or smaller, even more preferably 30% by mass or smaller. Amounts thereof smaller than the lower limit of that range are undesirable because they result in difficulties in obtaining the effect of improving conductivity. Amounts thereof exceeding the upper limit are undesirable because they lead to an increase in initial irreversible capacity.[Negative Electrode [1]: Electrode Production]

[0462] Negative electrode [1] may be produced by an ordinary method. It can be formed in the same manner as described above. The current collector, thickness ratio between the current collector and the active-material layer, electrode density, binder, electrode-plate orientation ratio, impedance, etc. also are the same as described above.<Negative Electrode [2]>

[0463] An explanation will be given below on negative electrode

[0464] for use in a lithium secondary battery of the invention, i.e., “a negative electrode containing as a negative-electrode active material at least one amorphous carbonaceous substance which, when examined by wide-angle X-ray diffractometry, has an interplanar spacing (d002) for the (002) planes of 0.337 nm or larger and a crystallite size (Lc) of 80 nm or smaller and which, in an examination by argon ion laser Raman spectroscopy, has a Raman R value defined as the ratio of the peak intensity at 1, 360 cm−1 to the peak intensity at 1, 580 cm−1 of 0.2 or higher”.[Negative Electrode [2]: Negative-Electrode Active Material]

[0465] The negative-electrode active material for use in negative electrode [2] is described below.

[0466] The negative-electrode active material for use in negative electrode [2] in a lithium secondary battery of the invention comprises an amorphous carbonaceous substance satisfying at least the following (a), (b), and (c).(a) The interplanar spacing (d002) for the (002) planes as determined by wide-angle X-ray diffractometry is 0.337 nm or larger.(b) The crystallite size (Lc) of the (002) planes as determined by wide-angle X-ray diffractometry is 80 nm or smaller.(c) The Raman R value defined as the ratio of the peak intensity at 1, 360 cm−1 to the peak intensity at 1, 580 cm−1 in an examination by argon ion laser Raman spectroscopy (hereinafter often referred to simply as “Raman R value”) is 0.2 or higher.

[0467] Although the negative-electrode active material for use in this invention comprises an amorphous carbonaceous substance satisfying at least (a), (b), and (c), the content of this amorphous carbonaceous substance in the whole negative-electrode active material is preferably 10% by mass or higher, especially preferably 50% by mass or higher. More preferably, the content thereof is 100%, i.e., this amorphous carbonaceous substance accounts for the whole negative-electrode active material. Negative-electrode active materials usable with this amorphous carbonaceous substance are not particularly limited, and known negative-electrode active materials such as artificial graphites and natural graphites may be used.[[Interplanar Spacing (d002) and Lc]]

[0468] The interplanar spacing (d002) for the (002) planes of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention, as determined by wide-angle X-ray diffractometry, is 0.337 nm or larger, preferably 0.34 nm or larger. The upper limit thereof is generally 0.39 nm or smaller, preferably 0.38 nm or smaller, more preferably 0.37 nm or smaller, even more preferably 0.36 or smaller, especially preferably 0.35 or smaller. In case where the interplanar spacing (d002) thereof exceeds the upper limit of that range, crystallinity decreases considerably and the resultant decrease in conductivity among the particles becomes not negligible. There are hence cases where the effect of improving short-time high-current-density charge / discharge characteristics is difficult to obtain. On the other hand, when the interplanar spacing (d002) thereof is smaller than the lower limit of that range, there are cases where crystallinity is too high and the effect of improving short-time high-current-density charge / discharge characteristics is difficult to obtain.

[0469] The term “interplanar spacing (d002) for the (002) planes as determined by wide-angle X-ray diffractometry” in the invention means the value of d (interplanar spacing) of the lattice planes ((002) planes) as determined by X-ray diffractometry by the method of the Japan Society for Promotion of Scientific Research.

[0470] On the other hand, the crystallite size (Lc) for the (002) planes of the amorphous carbonaceous substance as determined by X-ray diffractometry by the method of the Japan Society for Promotion of Scientific Research is 80 nm or smaller, preferably 35 nm or smaller, more preferably 20 nm or smaller, even more preferably 10 nm or smaller. The lower limit thereof is generally 0.1 nm or larger, preferably 1 nm or larger. When the Lc thereof is smaller than the lower limit of that range, there are cases where crystallinity decreases considerably and the resultant decrease in conductivity among the particles becomes not negligible and where the effect of improving suitability for short-time high-current-density charge / discharge is difficult to obtain. On the other hand, when the Lc thereof exceeds the upper limit of that range, there are cases where crystallinity is too high and the effect of improving suitability for short-time high-current-density charge / discharge is difficult to obtain.[[Raman R value]]

[0471] The Raman R value of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention should be 0.2 or higher, and is preferably 0.5 or higher, especially preferably 0.7 or higher, more preferably 0.8 or higher. The upper limit thereof is preferably 1.5 or lower, more preferably 1.2 or lower. In case where the Raman R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and suitability for charge decreases. There are hence cases where the effect of improving suitability for short-time high-current-density charge / discharge is difficult to obtain. On the other hand, in case where the Raman R value thereof exceeds the upper limit of that range, the surface of such particles has considerably reduced crystallinity, resulting in increased contact resistance among the particles. There are hence cases where the effect of improving suitability for short-time high-current-density charge / discharge is difficult to obtain.

[0472] The examination for a Raman spectrum is made with a Raman spectrometer (e.g., a Raman spectrometer manufactured by Japan Spectroscopic Co., Ltd.). In the examination, a sample is charged into a measuring cell by causing the sample to fall naturally into the cell and the surface of the sample in the cell is irradiated with argon ion laser light while rotating the cell in a plane perpendicular to the laser light. The Raman spectrum obtained is examined for the intensity IA of a peak PA at 1,580 cm−1 and the intensity IB of a peak PB at 1,360 cm−1. The ratio between these intensities R (R=IB / IA) is calculated and this ratio is defined as the Raman R value of the amorphous carbonaceous substance. Furthermore, the half-value width of the peak PA at 1,580 cm−1 in the Raman spectrum obtained is measured, and this value is defined as the Raman half-value width of the amorphous carbonaceous substance.

[0473] Conditions for this Raman spectroscopy are as follows.

[0474] Wavelength of argon ion laser: 514.5 nm

[0475] Laser power on sample: 15-25 mW

[0476] Resolution: 10-20 cm−1

[0477] Examination range: 1,100 cm−1 to 1,730 cm−1

[0478] Analysis for Raman R value and Raman half-value width: background processing

[0479] Smoothing: simple average; convolution, 5 points

[0480] The Raman half-value width at 1,580 cm−1 of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is not particularly limited. However, the half-value width thereof is generally 20 cm−1 or larger, preferably 25 cm−1 or larger. The upper limit thereof is generally 150 cm−1 or smaller, preferably 140 cm−1 or smaller. In case where the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and suitability for charge decreases. There is hence a possibility that the effect of improving suitability for short-time high-current-density charge / discharge might be difficult to obtain. On the other hand, in case where the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has considerably reduced crystallinity, resulting in increased contact resistance among the particles. There is hence a possibility that the effect of improving suitability for short-time high-current-density charge / discharge might be difficult to obtain. It is, however, noted that there are cases where the Raman half-value widths of some peaks cannot be judged because of the shapes of the peaks.

[0481] Although the amorphous carbonaceous substance for use in this invention satisfies the requirements concerning interplanar spacing (d002), crystallite size (Lc), and Raman R value described above, it is preferred from the standpoint of battery performance balance that the carbonaceous substance should further satisfy any one of the following requirements or simultaneously satisfy two or more thereof. It is especially preferred that any one of or two or more of true density, H / C value, O / C value, tap density, BET specific surface area, pore volume in the range of 0.01 μm to 1 μm, ash content, and volume-average particle diameter, among those requirements, should be simultaneously satisfied.[[True Density]]

[0482] The true density of the amorphous carbonaceous substance is generally 2.22 g / cm3 or lower, preferably 2.2 g / cm3 or lower, more preferably 2.1 g / cm3 or lower, even more preferably 2.0 g / cm3 or lower. The lower limit thereof is generally 1.4 g / cm3 or higher, preferably 1.5 g / cm3 or higher, more preferably 1.6 g / cm3 or higher, even more preferably 1.7 g / cm3 or higher, especially preferably 1.8 g / cm3 or higher. When the true density thereof is lower than the lower limit of that range, there are cases where this carbon has too low crystallinity, resulting in an increase in initial irreversible capacity. When the true density thereof exceeds the upper limit of that range, there are cases where this carbon has too high crystallinity and the effect of improving suitability for short-time high-current-density charge / discharge is difficult to obtain.

[0483] In the invention, true density is defined as one determined by the liquid-phase displacement method (pycnometer method) using butanol.[[O / C Value]]

[0484] The upper limit of the atomic amount ratio O / C value of the amorphous carbonaceous substance is generally 0.15 or lower, preferably 0.1 or lower, more preferably 0.05 or lower, even more preferably 0.03 or lower. The lower limit thereof is generally 0 or higher, preferably 0.01 or higher.

[0485] The term O / C value means the ratio of the molar concentration of oxygen atoms in the amorphous carbonaceous substance to the molar concentration of carbon atoms therein. It is an index to the amount of functional groups, such as carboxyl, phenolic hydroxyl, and carbonyl groups, present in the substance. An amorphous carbonaceous substance having a high O / C value frequently has an oxygen-containing functional group bonded to, e.g., an edge face of the carbon crystallite. When the O / C value of the amorphous carbonaceous substance exceeds the upper limit of that range, there are cases where irreversible capacity increases.[[H / C Value]]

[0486] The upper limit of the atomic amount ratio H / C value of the amorphous carbonaceous substance is generally 0.3 or lower, preferably 0.15 or lower, more preferably 0.1 or lower, even more preferably 0.08 or lower. The lower limit thereof is generally 0 or higher, preferably 0.01 or higher.

[0487] The term H / C value means the ratio of the molar concentration of hydrogen atoms in the amorphous carbonaceous substance to the molar concentration of carbon atoms therein. It is an index to the amount of hydrogen present in an edge face of the crystallite of the amorphous carbonaceous substance. An amorphous carbonaceous material having a high H / C value frequently has a large hydrogen amount relative to carbon amount in, e.g., edge faces of the carbon crystallites in the particle surface. When the H / C value of the amorphous carbonaceous substance exceeds the upper limit of that range, there are cases where irreversible capacity increases.

[0488] The terms “O / C value” and “H / C value” in the invention are values determined by the following CHN elemental analysis.[[CHN Elemental Analysis]]

[0489] An amorphous carbonaceous substance to be examined was vacuum-dried at 120° C. for about 15 hours and then dried at 100° C. for 1 hour on a hot plate in a dry box. Subsequently, a sample was taken out of the dried carbonaceous substance and placed in an aluminum cup in an argon atmosphere, and this sample was heat-treated. The carbon content, hydrogen content, and nitrogen content were determined respectively from the weights of the CO2 gas, H2O, and NO2 generated by the combustion. The inorganic content was determined from the weight of the residue which remained after the combustion. The oxygen amount was determined by subtracting the carbon content, hydrogen content, nitrogen content, and inorganic content from the whole weight. From these values, the number of moles of each element was calculated. The O / C value and H / C value were determined from the numbers of moles of the elements contained, using the following equations.O / C value=(number of moles of oxygen contained) / (number of moles of carbon contained)H / C value=(number of moles of hydrogen contained) / (number of moles of carbon contained)[[Tap Density]]

[0490] The tap density of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is preferably 0.1 g / cm3 or higher, more preferably 0.2 g / cm3 or higher, even more preferably 0.5 g / cm3 or higher, especially preferably 0.7 g / cm3 or higher. The upper limit thereof is preferably 1.4 g / cm3 or lower, more preferably 1.2 g / cm3 or lower, especially preferably 1.1 g / cm3 or lower. In case where the tap density thereof is lower than the lower limit of that range, this amorphous carbonaceous substance, when used in a negative electrode, is less apt to have a high loading density and has a reduced interparticle contact area. There are hence cases where interparticle resistance increases and short-time high-current-density chare / discharge characteristics decrease. On the other hand, in case where the tap density thereof exceeds the upper limit of that range, the electrode has too small an amount of interparticle interstices and a reduced amount of channels for the nonaqueous electrolyte. There are hence cases where short-time high-current-density chare / discharge characteristics decrease.

[0491] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the cell with the sample up to the brim, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining a density from the resultant volume of the sample and the weight thereof. This density is defined as the tap density.[[BET Specific Surface Area]]

[0492] The specific surface area of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention, as determined by the BET method, is preferably 0.1 m2 / g or larger, especially preferably 0.5 m2 / g or larger, more preferably 0.7 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is preferably 100 m2 / g or smaller, especially preferably 50 m2 / g or smaller, more preferably 25 m2 / g or smaller, even more preferably 15 m2 / g or smaller. When the BET specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this amorphous carbonaceous substance as a negative-electrode material is apt to result in poor lithium acceptance during charge and in lithium deposition on the electrode surface. On the other hand, when the specific surface area thereof exceeds the upper limit of that range, there are cases where use of this amorphous carbonaceous substance as a negative-electrode material is apt to result in enhanced reactivity with the nonaqueous electrolyte and enhanced gas evolution and a preferred battery is difficult to obtain.

[0493] BET specific surface area is defined as a value measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by preliminarily drying a sample at 350° C. for 15 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.[[Volume-Average Particle Diameter]]

[0494] The volume-average particle diameter of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention, which is defined as the volume-average particle diameter (median diameter) determined by the laser diffraction / scattering method, is preferably 1 μm or larger, especially preferably 3 μm or larger, more preferably 5 μm or larger, even more preferably 7 μm or larger. The upper limit thereof is generally 50 μm or smaller, preferably 40 μm or smaller, more preferably 30 μm or smaller, even more preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where irreversible capacity increases, leading to a loss in initial battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such an amorphous carbonaceous substance is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode plate is produced.[[Pore Volume]]

[0495] With respect to the pore volume of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention, the amount of interstices corresponding to a pore diameter of from 0.01 μm to 1 μm which include pores in the particles and particle surface irregularities formed by steps, as determined by mercury porosimetry (mercury intrusion method), may be 0.01 mL / g or larger and is preferably 0.05 mL / g or larger, more preferably 0.1 mL / g or larger. The upper limit thereof may be 0.6 mL / g or smaller and is preferably 0.4 mL / g or smaller, more preferably 0.3 mL / g or smaller. When the amount of interstices therein is larger than the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the amount thereof is smaller than the lower limit, there are cases where short-time high-current-density charge / discharge characteristics decrease and the effect of diminishing electrode expansion / contraction during charge / discharge is not obtained.

[0496] The total pore volume is preferably 0.1 mL / g or larger, more preferably 0.25 mL / g or larger. The upper limit thereof is generally 10 mL / g or smaller, preferably 5 mL / g or smaller, more preferably 2 mL / g or smaller. When the total pore volume exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary in electrode plate formation. When the total pore volume is smaller than the lower limit, there are cases where the dispersing effect of a thickener or binder in electrode plate formation is not obtained.

[0497] The average pore diameter of the amorphous carbonaceous substance is preferably 0.05 μm or larger, more preferably 0.1 μm or larger, even more preferably 0.5 μm or larger. The upper limit thereof is generally 50 μm or smaller, preferably 20 μm or smaller, more preferably 10 μm or smaller. When the average pore diameter thereof exceeds the upper limit of that range, there are cases where a large amount of a binder is necessary. When the average pore diameter thereof is smaller than the lower limit, there are cases where short-time high-current-density charge / discharge characteristics decrease.

[0498] As an apparatus for the mercury porosimetry is used a mercury porosimeter (Autopore 9520, manufactured by Micromeritics Corp.). About 0.2 g of a sample (negative-electrode material) is weighed out and placed in a powder cell and this cell is closed. The sample in the cell is pretreated by degassing it at room temperature under vacuum (50 μmHg or lower) for 10 minutes. Subsequently, the pressure in the cell is reduced to 4 psia (about 28 kPa) and mercury is introduced thereinto. The internal pressure is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia (about 280 MPa) and then lowered to 25 psia (about 170 kPa). The number of steps in the pressure elevation is 80 or larger. In each step, the amount of mercury intruded is measured after an equilibrium time of 10 seconds. A pore distribution is calculated from the mercury intrusion curve thus obtained, using the Washburn equation. For the calculation, the surface tension (γ) and contact angle (ψ) of mercury are taken as 485 dyne / cm and 140°, respectively. The average pore diameter used is the pore diameter corresponding to a cumulative pore volume of 50%.[[Ash Content]]

[0499] The ash content of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is preferably 1% by mass or lower, especially preferably 0.5% by mass or lower, more preferably 0.1% by mass or lower, based on the whole amorphous carbonaceous substance. The lower limit thereof is preferably 1 ppm by mass or higher. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the nonaqueous electrolyte during charge / discharge becomes not negligible. On the other hand, when the ash content thereof is lower than the lower limit of that range, there are cases where the production of this carbonaceous substance necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.[[Roundness]]

[0500] The roundness of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is generally 0.1 or higher, preferably 0.8 or higher, more preferably 0.85 or higher, especially preferably 0.9 or higher. With respect to the upper limit thereof, a particle having a roundness of 1 theoretically is a true sphere. In case where the roundness thereof is lower than the lower limit of that range, use of this negative-electrode active material results in a reduced loading and interparticle resistance increases. There are hence cases where short-time high-current-density charge / discharge characteristics decrease.

[0501] In the invention, roundness is defined by the following equation.Roundness=(length of periphery of equivalent circle having the same area as projected particle shape) / (actual length of periphery of projected particle shape)

[0502] The value of roundness is determined, for example, with a flow type particle image analyzer (e.g., FPIA, manufactured by Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene (20)) sorbitan monolaurate as a surfactant, propagating an ultrasonic wave of 28 kHz to the dispersion for 1 minute at an output of 60 W, subsequently examining particles having a particle diameter in the range of 3-40 μm with the analyzer having a detection range set at 0.6-400 μm, and averaging the measured values.[[Orientation Ratio]]

[0503] The orientation ratio of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is generally 0.005 or higher, preferably 0.01 or higher, more preferably 0.015 or higher. The upper limit thereof is theoretically 0.67 or lower. When the orientation ratio thereof is lower than the lower limit of that range, there are cases where short-time high-density charge / discharge characteristics decrease.

[0504] Orientation ratio is determined by X-ray diffractometry. The peaks attributable to the (110) diffraction and (004) diffraction of the carbon obtained by X-ray diffractometry are subjected to peak separation by fitting with asymmetric Pearson VII as a profile function. Thus, the integrated intensities of the (110) diffraction and (004) diffraction peaks are calculated. From the integrated intensities obtained, the ratio represented by (integrated intensity of (110) diffraction) / (integrated intensity of (004) diffraction) is calculated. This ratio is defined as the active-material orientation ratio.

[0505] Conditions for this X-ray diffractometry are as follows. Incidentally, “2θ” represents diffraction angle.

[0506] Target: Cu(Kα line) graphite monochromator

[0507] Slit:

[0508] Divergence slit=1 degree

[0509] Receiving slit=0.1 mm

[0510] Scattering slit=1 degree

[0511] Examination range and step angle / measuring time:

[0512] (110) plane: 76.5° 20 78.5° .01° / 3 sec

[0513] (004) plane: 53.5°≤20 56.0° .01° / 3 sec[[Aspect Ratio]]

[0514] The aspect ratio of the amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is theoretically 1 or higher. The upper limit thereof is generally 10 or lower, preferably 8 or lower, more preferably 5 or lower. When the aspect ratio thereof exceeds the upper limit, there are cases where this amorphous carbonaceous material causes streak lines in electrode plate formation and an even coating surface cannot be obtained, resulting in a decrease in short-time high-current-density charge / discharge characteristics.

[0515] The aspect ratio of a particle is represented by A / B, wherein A is the length of the longest axis of the particle in a three-dimensional examination and B is the length of the shortest axis perpendicular to that axis. The examination of particles is made with a scanning electron microscope capable of enlargement examination. Fifty are arbitrarily selected from particles fixed to an edge face of a metal having a thickness of 50 μm or smaller, and each particle is examined for A and B while rotating or inclining the stage to which the sample is fixed. The average of the A / B values is determined.

[0516] The amorphous carbonaceous substance contained as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention preferably is an amorphous carbonaceous substance selected from the following (1) to (4):(1) one obtained by further heat-treating a carbonization product selected from the group consisting of coal coke, petroleum coke, furnace black, acetylene black, and pitch-derived carbon fibers;(2) an organic substance selected from the group consisting of pitch materials, aromatic hydrocarbons, nitrogen-ring compounds, sulfur-ring compounds, polyphenylenes, organic synthetic polymers, natural polymers, thermoplastic resins, and thermosetting resins and / or a product of the pyrolysis of the organic substance and / or a substance obtained by further heat-treating the pyrolysis product;(3) a product of the pyrolysis of a solution obtained by dissolving the organic substance (2) in a low-molecular organic solvent and / or a substance obtained by further heat-treating the pyrolysis product; and(4) a product of the carbonization of a gas containing an organic substance.

[0517] The substance in (2) is not particularly limited as long as it is capable of carbonization. Examples thereof include the following organic substances: pitch materials; aromatic hydrocarbons such as acenaphthylene, decacyclene, anthracene, and phenanthrene; nitrogen-ring compounds such as phenazine and acridine; sulfur-ring compounds such as thiophene and bithiophene; polyphenylenes such as biphenyl and terphenyl; poly(vinylchloride), poly(vinylalcohol),poly(vinylbutyral), substances obtained by crosslinking or insolubilizing these compounds; nitrogen-containing organic polymers such as polyacrylonitrile and polypyrrole; sulfur-containing organic polymers such as polythiophene; organic polymers such as polystyrene; natural polymers such as polysaccharides represented by cellulose, lignin, mannan, poly(galacturonic acid), chitosan, and saccharose; thermoplastic resins such as poly(phenylene sulfide) and poly(phenylene oxide); thermosetting resins such as furfuryl alcohol resins, phenol-formaldehyde resins, and imide resins; and solutions obtained by dissolving any of such organic substances in a low-molecular organic solvent, e.g., benzene, toluene, xylene, quinoline, or n-hexane. Examples thereof further include gases capable of carbonization and containing any of those organic substances.

[0518] Preferred of these are pitch materials. This is because pitch materials have a high actual carbon ratio and, hence, can give a material in high yield. The term “pitch material” in this description means a substance which is a pitch or a similar material and can be carbonized or graphitized by an appropriate treatment. Examples of the pitch materials which can be used include tars, heavy oils, and pitches. Examples of the tars include coal tar and petroleum tar. Examples of the heavy oils include petroleum-derived heavy oils such as catalytically cracked oil, thermally cracked oil, topping residues, and vacuum distillation residues. Examples of the pitches include coal tar pitch, petroleum pitch, and synthetic pitch. Of these, coal tar pitch is preferred because it has high aromaticity. Anyone of these pitch materials may be used alone, or any desired two or more thereof may be used in combination in any desired proportion.

[0519] With respect to (3), preferred examples thereof include a carbonization product obtained from a precursor which is a product of the pyrolysis of a solution of the organic substance (2) in a low-molecular organic solvent such as benzene, toluene, xylene, quinoline, or n-hexane.

[0520] With respect to (4), examples include hydrocarbon compounds such as methane, ethane, propane, benzene, acetylene, and ethylene; and carbon monoxide.

[0521] To conduct a crosslinking treatment is also preferred. A crosslinking treatment is conducted for the purpose of giving a crosslinked pitch material or the like which, when heat-treated, gives a carbonaceous material which is less apt to be graphitized. By conducting this treatment, charge capacity per unit mass can be increased.

[0522] Examples of the crosslinking treatment include: a crosslinking treatment in which a vinyl monomer such as divinylbenzene, trivinylbenzene, diallyl phthalate, ethylene glycol dimethacrylate, or N,N-methylenebisacrylamide is used in combination with a radical polymerization initiator such as a,a′-azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), lauryl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, or hydrogen peroxide; and an oxidizing agent treatment in which an oxidizing gas such as oxygen, ozone, or nitrogen dioxide or an oxidizing liquid such as sulfuric acid, nitric acid, or an aqueous hydrogen peroxide solution is used. One example of methods for such crosslinking treatments comprises mixing a pitch material kept at 50° C.-400° C. with a crosslinking agent, oxidizing agent, etc. to treat the material.[[Li-NMR Shift]]

[0523] When an amorphous carbonaceous substance for use as the negative-electrode active material of negative electrode [2] in a lithium secondary battery of the invention is charged to a fully charged state and subjected, in this state, to 7Li-NMR analysis, then there are cases where a main resonance peak is observed in a position shifted by 80-200 ppm to the low-magnetic-field side from the resonance line for LiCl as a reference. This amorphous carbonaceous substance has an increased capacity per unit mass. It is therefore preferred to use an amorphous carbonaceous substance which has undergone a crosslinking treatment.[[Processes for Producing Amorphous Carbonaceous Substance]]

[0524] Processes for producing the amorphous carbonaceous substance described above are not particularly limited unless they depart from the spirit of the invention. Examples thereof include several processes. For producing the amorphous carbonaceous substance, it is necessary to conduct a heat treatment one or more times. However, a heat treatment may be conducted in two or more portions. It is also preferred to conduct any of various treatments before or after a heat treatment and / or during the heat treatment. Examples of the various treatments include pulverization, classification, and the crosslinking treatment described above. The pulverization and classification treatments can be conducted in any stage, i.e., before, after, or during the heat treatment, as long as the carbonaceous substance is solid. In the case of the crosslinking treatment, it preferably is conducted before or during the heat treatment. By conducting these treatments, the specific surface area of the negative-electrode active material can be regulated and the capacity thereof per unit mass can be increased.

[0525] Apparatus usable for the pulverization before the heat treatment are not particularly limited. Examples of coarse pulverizers include shearing mills, jaw crushers, impact crushers, and cone crushers. Examples of intermediate pulverizers include roll crushers and hammer mills. Examples of fine pulverizers include ball mills, oscillating mills, pin mills, stirring mills, and jet mills.

[0526] Apparatus usable for the heat treatment of a raw material are not particularly limited. For example, use can be made of a reaction vessel such as a shuttle furnace, tunnel kiln, electric furnace, Riedhammer furnace, rotary kiln, or autoclave, a coker (heat-treatment vessel for coke production), a direct voltage application type electric furnace, or the like. During the treatment of a raw material, stirring may be conducted according to need.

[0527] Temperature conditions in the case of conducting the heat treatment are not particularly limited. However, the temperature is generally 600° C. or higher, preferably 900° C. or higher. The upper limit thereof is generally 2,500° C. or lower, preferably 1,300° C. or lower. In case where the temperature conditions are lower than the lower limit of that range, there is a possibility that crystallinity might become too low, resulting in an increase in irreversible capacity. On the other hand, in case where the temperature exceeds the upper limit of that range, there is a possibility that crystallinity might become too high, resulting in reduced short-time high-current-density charge / discharge characteristics.

[0528] The amorphous carbonaceous substance which has undergone the heat treatment may be subjected to pulverization and classification according to the size of the aggregates or particles. Apparatus usable for the pulverization are not particularly limited. Examples of crushers include shearing mills, jaw crushers, impact crushers, and cone crushers. Examples of intermediate pulverizers include roll crushers and hammer mills. Examples of pulverizers include ball mills, oscillating mills, pin mills, stirring mills, and jet mills. Apparatus usable for the classification are not particularly limited. However, in the case of dry sieving, use can be made of a rotary sieve, swing sieve, revolving sieve, vibrating sieve, or the like. In the case of dry air classification, use can be made of a gravitational classifier, inertial classifier, or centrifugal classifier (e.g., classifier or cyclone). Furthermore, for wet sieving, use can be made of a mechanical wet classifier, hydraulic classifier, sedimentation classifier, centrifugal wet classifier, or the like.[Negative Electrode [2]: Electrode Production]

[0529] Negative electrode [2] may be produced by an ordinary method. It can be formed in the same manner as described above. The current collector, thickness ratio between the current collector and the active-material layer, electrode density, binder, electrode-plate orientation ratio, impedance, etc. also are the same as described above.<Negative Electrode [3]>

[0530] An explanation will be given below on negative electrode

[0531] for use in a lithium secondary battery of the invention, i.e., “a negative electrode containing as a negative-electrode active material a titanium-containing metal oxide capable of occluding and releasing lithium”.[Negative Electrode [3]: Negative-Electrode Active Material]

[0532] The negative-electrode active material for use in negative electrode [3] is described below.[[Constitution of Negative-Electrode Active Material]]

[0533] The negative-electrode active material for use in negative electrode [3] in a lithium secondary battery of the invention contains a titanium-containing metal oxide capable of occluding and releasing lithium. Preferred of such metal oxides is a composite oxide of lithium and titanium (hereinafter abbreviated to “lithium-titanium composite oxide”). Furthermore, the metal oxide preferably is a titanium-containing metal oxide having a spinel structure. A metal oxide simultaneously satisfying these, i.e., a lithium-titanium composite oxide having a spinel structure, is especially preferred because use of this composite oxide in the negative-electrode active material for a lithium secondary battery is effective in considerably reducing output resistance.

[0534] Also preferred are lithium-titanium composite oxides in which the lithium or titanium has been replaced by one or more other metallic elements, e.g., at least one element selected from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

[0535] The metal oxide preferably is a lithium-titanium composite oxide represented by general formula (1) wherein 0.7≤x≤1.5, 1.5≤y≤2.3, and 0≤z≤1.6, because the structure thereof is stable during lithium ion doping / undoping.LixTiyMzO4  (1)[In general formula (1), M represents at least one element selected from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.]

[0536] Of the compositions represented by general formula (1), the following structures are especially preferred because they bring about a satisfactory balance among battery performances. Structures represented by general formula (1), i.e., LixTiyM2O4, wherein(a) 1.2≤x≤1.4, 1.5≤y≤1.7, and z=0;(b) 0.9≤x≤1.1, 1.9≤y≤2.1, and z=0; or(c) 0.7≤x≤0.9, 2.1≤y≤2.3, and z=0.

[0537] Especially preferred typical compositions of those compounds are: Li4 / 3Ti5 / 3O4 for (a), Li1Ti2O4 for (b), and Li4 / 5Ti11 / 5O4 for (c).

[0538] Preferred examples of the structure wherein z #0 include Li4 / 3Ti4 / 3Al1 / 3O4.[[Properties, Shape, etc. of Negative-Electrode Active Material]]

[0539] It is preferred that the negative-electrode active material for use in negative electrode [3] in a lithium secondary battery of the invention should satisfy at least one of the following properties besides the requirements described above. Especially preferably, the negative-electrode active material simultaneously satisfies two or more of the following properties besides the requirements described above.[[[BET Specific Surface Area]]]

[0540] The specific surface area of the titanium-containing metal oxide for use as the negative-electrode active material in negative electrode [3] in a lithium secondary battery of the invention, as determined by the BET method, is preferably 0.5 m2 / g or larger, more preferably 0.7 m2 / g or larger, especially preferably 1.0 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is preferably 200 m2 / g or smaller, more preferably 100 m2 / g or smaller, especially preferably 50 m2 / g or smaller, even more preferably 25 m2 / g or smaller. When the BET specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this metal oxide as a negative-electrode material results in a reduced reaction area available for contact with the nonaqueous electrolyte and in an increase in output resistance. On the other hand, in case where the BET specific surface area thereof exceeds the upper limit of that range, the proportion of surfaces and edge faces of crystals of the titanium-containing metal oxide increases and this causes crystal deformation. There are hence cases where irreversible capacity becomes not negligible and a preferred battery is difficult to obtain.

[0541] BET specific surface area is defined as a value measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by preliminarily drying a sample at 350° C. for 15 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere pressure.[[[Volume-Average Particle Diameter]]]

[0542] The volume-average particle diameter (secondary-particle diameter in the case where the primary particles have aggregated to form secondary particles) of the titanium-containing metal oxide for use as the negative-electrode active material in negative electrode [3] in a lithium secondary battery of the invention, which is defined as the volume-average particle diameter (median diameter) determined by the laser diffraction / scattering method, is preferably 0.1 μm or larger, more preferably 0.5 μm or larger, even more preferably 0.7 μm or larger. The upper limit thereof is generally 50 μm or smaller, preferably 40 μm or smaller, more preferably 30 μm or smaller, even more preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where a large amount of a binder is necessary in electrode production and this results in a decrease in battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such a metal oxide is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode plate is produced.[[[Average Primary-Particle Diameter]]]

[0543] In the case where the primary particles have aggregated to form secondary particles, the average primary-particle diameter of the titanium-containing metal oxide for use as the negative-electrode active material in a lithium secondary battery of the invention is preferably 0.01 μm or larger, more preferably 0.05 μm or larger, even more preferably 0.1 μm or larger, most preferably 0.2 μm or larger. The upper limit thereof is preferably 2 μm or smaller, more preferably 1.6 μm or smaller, even more preferably 1.3 μm or smaller, most preferably 1 μm or smaller. In case where the average primary-particle diameter thereof exceeds the upper limit, spherical secondary particles are difficult to form and this adversely influences powder loading or results in a considerably reduced specific surface area. There may hence be a high possibility that battery performances such as output characteristics might decrease. Conversely, when the average primary-particle diameter thereof is smaller than the lower limit, crystal growth is usually insufficient and, hence, there are cases where this positive-electrode active material poses a problem, for example, that charge / discharge reversibility is poor.

[0544] Primary-particle diameter is determined through an examination with a scanning electron microscope (SEM). Specifically, arbitrarily selected 50 primary-particle images in a photograph having a magnification capable of particle observation, e.g., 10,000-100,000 diameters, each are examined for the length of the longest segment of a horizontal line which extends across the primary-particle image from one side to the other side of the boundary. These measured lengths are averaged, whereby the average value can be determined.[[[Shape]]]

[0545] The particle shape of the titanium-containing metal oxide for use in negative electrode [3] in a lithium secondary battery of the invention may be any of massive, polyhedral, spherical, ellipsoidal, platy, acicular, columnar, and other shapes such as those in common use. Preferred of these is one in which the primary particles have aggregated to form secondary particles and these secondary particles have a spherical or ellipsoidal shape. In electrochemical elements, the active material in each electrode usually expands / contracts with the charge / discharge of the element and, hence, deterioration is apt to occur, such as active-material breakage and conduction path breakage, due to the stress caused by the expansion / contraction. Because of this, a positive-electrode active material in which the primary particles have aggregated to form secondary particles is preferable to an active material composed of primary particles only since the particles in the former active material relieve the stress caused by expansion / contraction to prevent deterioration. Furthermore, particles of a spherical or ellipsoidal shape are preferable to particles showing axial orientation, e.g., platy ones, because the former particles are less apt to orient during electrode molding and hence this electrode is reduced in expansion / contraction during charge / discharge, and because these particles are apt to be evenly mixed with a conductive material in electrode production.[[[Tap Density]]]

[0546] The tap density of the titanium-containing metal oxide for use as the negative-electrode active material in negative electrode [3] in a lithium secondary battery of the invention is preferably 0.05 g / cm3 or higher, more preferably 0.1 g / cm3 or higher, even more preferably 0.2 g / cm3 or higher, especially preferably 0.4 g / cm3 or higher. The upper limit thereof is preferably 2.8 g / cm3 or lower, more preferably 2.4 g / cm3 or lower, especially preferably 2 g / cm3 or lower. In case where the tap density thereof is lower than the lower limit of that range, this metal oxide, when used in a negative electrode, is less apt to have a high loading density and has a reduced interparticle contact area. There are hence cases where interparticle resistance increases and output resistance increases. On the other hand, in case where the tap density thereof exceeds the upper limit of that range, the electrode has too small an amount of interparticle interstices and a reduced amount of channels for the nonaqueous electrolyte. There are hence cases where output resistance increases.

[0547] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the cell with the sample up to the brim, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and determining a density from the resultant volume of the sample and the weight thereof. This density is defined as the tap density.[[[Roundness]]]

[0548] The roundness of the titanium-containing metal oxide for use as the negative-electrode active material of negative electrode [3] in a lithium secondary battery of the invention is generally 0.10 or higher, preferably 0.80 or higher, more preferably 0.85 or higher, especially preferably 0.90 or higher. With respect to the upper limit thereof, a particle having a roundness of 1 theoretically is a true sphere. In case where the roundness thereof is lower than the lower limit of that range, use of this negative-electrode active material results in a reduced loading and interparticle resistance increases. There are hence cases where short-time high-current-density charge / discharge characteristics decrease.

[0549] In the invention, roundness is defined by the following equation.Roundness=(length of periphery of equivalent circle having the same area as projected particle shape) / (actual length of periphery of projected particle shape)

[0550] The value of roundness is determined with a flow type particle image analyzer (e.g., FPIA, manufactured by Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene (20)) sorbitan monolaurate as a surfactant, propagating an ultrasonic wave of 28 kHz to the dispersion for 1 minute at an output of 60 W, and then examining particles having a particle diameter in the range of 3-40 μm with the analyzer having a detection range set at 0.6-400 μm.[[[Aspect Ratio]]]

[0551] The aspect ratio of the titanium-containing metal oxide for use as the negative-electrode active material of negative electrode [3] in a lithium secondary battery of the invention is theoretically 1 or higher. The upper limit thereof may be 5 or lower and is preferably 4 or lower, more preferably 3 or lower, even more preferably 2 or lower. When the aspect ratio thereof exceeds the upper limit, there are cases where this metal oxide causes streak lines in electrode plate formation and an even coating surface cannot be obtained, resulting in a decrease in short-time high-current-density charge / discharge characteristics.

[0552] The aspect ratio of a particle is represented by A / B, wherein A is the length of the longest axis of the particle in a three-dimensional examination and B is the length of the shortest axis perpendicular to that axis. The examination of particles is made with a scanning electron microscope capable of enlargement examination. Fifty are arbitrarily selected from particles fixed to an edge face of a metal having a thickness of 50 μm or smaller, and each particle is examined for A and B while rotating or inclining the stage to which the sample is fixed. The average of the A / B values is determined.[[Processes for Producing Negative-Electrode Active Material]]

[0553] Processes for producing the negative-electrode active material of negative electrode [3] in a lithium secondary battery of the invention are not particularly limited unless they depart from the spirit of the invention. Examples thereof include several processes, and processes in general use for producing inorganic compounds may be employed. For example, use may be made of a method in which a titanium source, e.g., titanium oxide, is evenly mixed with an Li source, e.g., LiOH, Li2CO3, or LiNO3, and optionally with a source of other element (s) and this mixture is heat-treated at a high temperature to obtain the active material. Especially for producing spherical or ellipsoidal active materials, various techniques are usable. Examples thereof include: a method which comprises dissolving or pulverizing / dispersing a titanium source, e.g., titanium oxide, optionally together with a source of other element(s) in a solvent, e.g., water, regulating the pH of the solution or dispersion with stirring to produce a spherical precursor, recovering and optionally drying the precursor, subsequently adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; a method which comprises dissolving or pulverizing / dispersing a titanium source, e.g., titanium oxide, optionally together with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, adding thereto an Li source, e.g., LiOH, Li2CO3, or LiNO3, and heat-treating the mixture at a high temperature to obtain the active material; and a method which comprises dissolving or pulverizing / dispersing a titanium source, e.g., titanium oxide, together with an Li source, e.g., LiOH, Li2CO3, or LiNO3, and optionally with a source of other element(s) in a solvent, e.g., water, drying and forming the solution or dispersion with a spray dryer or the like to obtain a spherical or ellipsoidal precursor, and heat-treating the precursor at a high temperature to obtain the active material.

[0554] In those steps, one or more of elements other than Ti, such as, e.g., Al, Mn, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, C, Si, Sn, and Ag, can be caused to be present in the titanium-containing metal oxide structure and / or present so as be in contact with the titanium-containing oxide. The incorporation of such elements can be used for regulating the operating voltage and capacity of the battery.[Negative Electrode [3]: Electrode Production]

[0555] Negative electrode [3] may be produced by an ordinary method. It can be formed in the same manner as described above. The current collector, thickness ratio between the current collector and the active-material layer, electrode density, binder, electrode-plate orientation ratio, impedance, etc. also are the same as described above.<Negative Electrode [4]>

[0556] An explanation will be given below on negative electrode

[0557] for use in a lithium secondary battery of the invention, i.e., “a negative electrode containing as a negative-electrode active material a carbonaceous substance having a roundness of 0.85 or higher and a surface functional-group amount O / C value of from 0 to 0.01”.[Negative Electrode [4]: Negative-Electrode Active Material]

[0558] The negative-electrode active material for use in negative electrode [4] is described below.

[0559] The negative-electrode active material for use in negative electrode [4] in a lithium secondary battery of the invention comprises a carbonaceous substance satisfying at least the following requirements (a) and (b).(a) The roundness is 0.85 or higher.(b) The surface functional-group amount O / C value is from 0 to 0.01.

[0560] Details of the carbonaceous substance for use in this invention are explained below.[[Roundness]]

[0561] The roundness of the carbonaceous substance is generally 0.85 or higher, preferably 0.87 or higher, more preferably 0.89 or higher, especially preferably 0.92 or higher. With respect to the upper limit thereof, a particle having a roundness of 1 theoretically is a true sphere. In case where the roundness thereof is lower than the lower limit of that range, use of this negative-electrode active material results in a reduced loading and compaction in negative-electrode formation is difficult and leads to particle breakage. There is hence a possibility that those inner parts of the particles which have poor high-temperature storability at a low state of charge are apt to be exposed.

[0562] In the invention, roundness is defined by the following equation.Roundness=(length of periphery of equivalent circle having the same area as projected particle shape) / (actual length of periphery of projected particle shape)

[0563] The value of roundness is determined, for example, with a flow type particle image analyzer (e.g., FPIA, manufactured by Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene (20)) sorbitan monolaurate as a surfactant, propagating an ultrasonic wave of 28 kHz to the dispersion for 1 minute at an output of 60 W, and then examining particles having a particle diameter in the range of 3-40 μm with the analyzer having a detection range set at 0.6-400 m.[[O / C Value]]

[0564] The surface functional-group amount O / C value of the carbonaceous substance should be from 0 to 0.01. The upper limit of the O / C value thereof is preferably 0.005 or lower. The closer the O / C value to 0, the more the carbonaceous substance is preferred.

[0565] The term O / C value in the invention indicates the proportion of the amount of surface functional groups determined by X-ray photoelectron spectroscopy (XPS). In case where the surface functional-group amount O / C value exceeds the upper limit of that range, the particle surface has an increased amount of a functional group and the SEI film which is formed on the surface during charge in the presence of a specific compound does not have sufficient stability. There are hence cases where high-temperature storability at a low state of charge decreases.

[0566] Surface functional-group amount O / C value means the ratio of the concentration of oxygen atoms present on the surface of, e.g., a graphite material, to the concentration of carbon atoms present thereon. It is an index to the amount of functional groups, such as carboxyl, phenolic hydroxyl, and carbonyl groups, present on the surface. When a carbon material has a high surface functional-group amount O / C value, this frequently means that an oxygen-containing surface functional group is bonded to, e.g., edge faces of the carbon crystallites in the particle surface. Incidentally, the surface functional-group amount O / C value of a graphite material is determined by determining the areas of C1s and O1s spectral peaks in X-ray photoelectron spectroscopy and calculating the carbon-oxygen atomic concentration ratio O / C (O atom concentration / C atom concentration) from the peak areas. Although specific procedures of the determination are not particularly limited, an example is as follows.

[0567] An X-ray photoelectron spectrometer (e.g., ESCA, manufactured by ULVAC-PHI, INC.) is used for the X-ray photoelectron spectroscopy. A sample to be examined (here, a graphite material) is placed on a sample table so as to form a flat surface. An aluminum Kα line is used as an X-ray source to obtain a spectrum having C1s (280-300 eV) and O1s (525-545 eV) peaks in the multiplex mode. Charge correction is made, with the top of the C1s peak being at 284.3 eV, and the areas of the C1s and O1s spectral peaks are determined. Furthermore, these areas are multiplied by the apparatus sensitivity coefficient to calculate the concentrations of surface C atoms and surface O atoms. From the O and C atom concentrations obtained, the atomic concentration ratio O / C (O atom concentration / C atom concentration) is calculated. This ratio is defined as the surface functional-group amount O / C value of the graphite material.

[0568] Although the carbonaceous substance for use in this invention satisfies the requirements concerning “roundness” and “surface functional-group amount O / C value” described above, it is preferred from the standpoint of battery performance balance that the carbonaceous substance should further satisfy any one of the following requirements or simultaneously satisfy two or more thereof. It is preferred that any one of or two or more of tap density, Raman R value, and volume-average particle diameter, among those requirements, should be simultaneously satisfied.[[Tap Density]]

[0569] The tap density of the carbonaceous substance is generally 0.55 g / cm3 or higher, preferably 0.7 g / cm3 or higher, more preferably 0.8 g / cm3 or higher, especially preferably 0.9 g / cm3 or higher. The upper limit thereof is preferably 2 g / cm3 or lower, more preferably 1.8 g / cm3 or lower, especially preferably 1.6 g / cm3 or lower. In case where the tap density thereof is lower than the lower limit of that range, this carbonaceous substance, when used in a negative electrode, is less apt to have a high loading density. There are hence cases where a high-capacity battery cannot be obtained. On the other hand, in case where the tap density thereof exceeds the upper limit of that range, the electrode has too small an amount of interparticle interstices and electrical conductivity among the particles is difficult to secure. There are hence cases where preferred battery characteristics are difficult to obtain.

[0570] In the invention, the tap density of a sample is determined by dropping the sample through a sieve having an opening size of 300 μm into a 20-cm3 tapping cell to fill the cell with the sample up to the brim, subsequently conducting tapping operations 1,000 times over a stroke length of 10 mm using a powder densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), and calculating a density from the resultant volume of the sample and the weight thereof. This value is defined as the tap density.[[Raman R Value, Half-Value Width]]

[0571] The R value of the carbonaceous substance, as determined by argon ion laser Raman spectroscopy, is generally 0.001 or higher, preferably 0.01 or higher. The upper limit thereof is generally 0.2 or lower, preferably 0.18 or lower, more preferably 0.15 or lower. When the R value thereof is lower than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. On the other hand, in case where the R value thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and the SEI film which is formed on the surface during charge in the presence of a specific compound is not sufficiently stable. There are hence cases where high-temperature storability at a low state of charge decreases.

[0572] The Raman half-value width around 1,580 cm−1 of the carbonaceous substance is not particularly limited. However, the half-value width thereof is generally 10 cm−1 or larger, preferably 15 cm−1 or larger. The upper limit thereof is generally 35 cm−1 or smaller, preferably 30 cm−1 or smaller. When the Raman half-value width thereof is smaller than the lower limit of that range, the surface of such particles has too high crystallinity and there are cases where the number of intercalation sites into which lithium comes with charge / discharge decreases. Namely, there are cases where suitability for charge decreases. On the other hand, when the Raman half-value width thereof exceeds the upper limit of that range, the surface of such particles has reduced crystallinity and the SEI film which is formed on the surface during charge in the presence of a specific compound is not sufficiently stable. There are hence cases where high-temperature storability at a low state of charge decreases.

[0573] The examination for a Raman spectrum is made with a Raman spectrometer (e.g., a Raman spectrometer manufactured by Japan Spectroscopic Co., Ltd.). In the examination, a sample is charged into a measuring cell by causing the sample to fall naturally into the cell and the surface of the sample in the cell is irradiated with argon ion laser light while rotating the cell in a plane perpendicular to the laser light. The Raman spectrum obtained is examined for the intensity IA of a peak PA around 1,580 cm−1 and the intensity IB of a peak PB around 1,360 cm−1. The ratio between these intensities R (R=IB / IA) is calculated and this ratio is defined as the Raman R value of the carbon material. Furthermore, the half-value width of the peak PA around 1,580 cm−1 in the Raman spectrum obtained is measured, and this value is defined as the Raman half-value width of the carbonaceous substance.

[0574] Conditions for this Raman spectroscopy are as follows.

[0575] Wavelength of argon ion laser: 514.5 nm

[0576] Laser power on sample: 15-25 mW

[0577] Resolution: 10-20 cm−1

[0578] Examination range: 1,100-1,730 cm−1

[0579] Analysis for R value and half-value width: background processing

[0580] Smoothing: simple average; convolution, 5 points[[Volume-Average Particle Diameter]]

[0581] With respect to the volume-average particle diameter of the carbonaceous substance, the volume-average particle diameter (median diameter) thereof as determined by the laser diffraction / scattering method is generally 1 μm or larger, preferably 3 μm or larger, more preferably 5 μm or larger, even more preferably 7 μm or larger. The upper limit thereof is generally 50 μm or smaller, preferably 40 μm or smaller, more preferably 30 μm or smaller, even more preferably 25 μm or smaller. When the volume-average particle diameter thereof is smaller than the lower limit of that range, there are cases where irreversible capacity increases, leading to a loss in initial battery capacity. When the volume-average particle diameter thereof exceeds the upper limit of that range, there are cases where such a carbonaceous substance is undesirable from the standpoint of battery production because an uneven coating surface is apt to result when an electrode plate is produced.

[0582] Furthermore, in the volume-based particle diameter distribution thereof, the ratio of the 90% particle diameter to the 10% particle diameter (d90 / d10) may be 1.2 or higher and is preferably 1.5 or higher, more preferably 1.7 or higher. The upper limit thereof may be 8 or lower and is preferably 5 or lower, more preferably 4 or lower, even more preferably 3 or lower.

[0583] Volume-average particle diameter in the invention is defined as the median diameter determined by dispersing the carbon powder in a 0.2% by mass aqueous solution (about 1 mL) of polyoxyethylene (20) sorbitan monolaurate as a surfactant and examining the dispersion with a laser diffraction type particle size distribution analyzer (e.g., LA-700, manufactured by HORIBA, Ltd.). With respect to the 90% particle diameter / 10% particle diameter ratio (d90 / d10), the volume-based 90% particle diameter and 10% particle diameter are determined in the same manner and the ratio (d90 / d10) can be calculated therefrom.[[X-Ray Parameter]]

[0584] The carbonaceous substance preferably has a value of d (interplanar spacing) for the lattice planes (002), as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, of 0.335 nm or larger. The upper limit of the value of d is 0.340 nm or smaller and is preferably 0.337 nm or smaller. When the value of d is too large, there are cases where such particles have reduced crystallinity to increase initial irreversible capacity. On the other hand, 0.335 is a theoretical value for graphites. Furthermore, the crystallite size (Lc) of this carbon material, as determined by X-ray diffractometry in accordance with the method of the Japan Society for Promotion of Scientific Research, is generally 30 nm or larger, preferably 50 nm or larger, more preferably 80 nm or larger. In case where the Lc thereof is smaller than the lower limit of that range, there is a possibility that such particles might have reduced crystallinity to increase initial irreversible capacity.[[Ash Content]]

[0585] The ash content of the carbonaceous substance is preferably 1% by mass or lower, especially 0.5% by mass or lower, in particular 0.1% by mass or lower, based on the whole carbonaceous substance. The lower limit of the ash content thereof is preferably at least 1 ppm by mass of the whole carbonaceous substance. When the ash content thereof exceeds the upper limit of that range, there are cases where battery performance deterioration caused by reaction with the electrolyte during charge / discharge becomes not negligible. When the ash content thereof is lower than the lower limit of that range, there are cases where the production of this carbonaceous substance necessitates much time and energy and an apparatus for pollution prevention, resulting in an increase in cost.[[BET Specific Surface Area]]

[0586] The specific surface area of the carbonaceous substance, as determined by the BET method, is generally 0.1 m2 / g or larger, preferably 0.7 m2 / g or larger, more preferably 1 m2 / g or larger, even more preferably 1.5 m2 / g or larger. The upper limit thereof is generally 100 m2 / g or smaller, preferably 25 m2 / g or smaller, more preferably 15 m2 / g or smaller, even more preferably 10 m2 / g or smaller. When the specific surface area thereof is smaller than the lower limit of that range, there are cases where use of this carbonaceous substance as a negative-electrode material is apt to result in poor lithium acceptance during charge and in lithium deposition on the electrode surface. On the other hand, when the specific surface area thereof exceeds the upper limit of that range, there are cases where use of this carbonaceous substance as a negative-electrode material is apt to result in enhanced reactivity with the electrolyte and enhanced gas evolution and a preferred battery is difficult to obtain.

[0587] BET specific surface area is defined as a value measured with a surface area meter (e.g., a fully automatic surface area measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by preliminarily drying a sample at 350° C. for 15 minutes in a nitrogen stream and then measuring the specific surface area thereof by the gas-flowing nitrogen adsorption BET one-point method using a nitrogen / helium mixture gas precisely regulated so as to have a nitrogen pressure of 0.3 relative to atmosphere press...

Claims

1. A lithium secondary battery at least comprising:an electrode group comprising a positive electrode, a negative electrode, and a microporous film separator interposed between the electrodes; and a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt contained therein, the electrode group and the nonaqueous electrolyte being held in a battery case, and the positive electrode and the negative electrode each comprising a current collector and, formed thereon, an active-material layer containing an active material capable of occluding / releasing a lithium ion,wherein the nonaqueous electrolyte is a nonaqueous electrolyte which contains at least one compound selected from the group consisting of cyclic siloxane compounds represented by the following general formula (1):wherein R1 and R2 may be the same or different and represent an organic group which has 1-12 carbon atoms and n represents an integer of 3-10,fluorosilane compounds represented by the following general formula (2):SiFxR3pR4qR5r,  (2)wherein R3 to R5 may be the same or different and represent an organic group which has 1-12 carbon atoms; x represents an integer of 1-3; and p, q, and r each represents an integer of 0-3, provided that 1≤p+q+r≤3,compounds represented by the following general formula (3):wherein R6 to R8 may be the same or different and represent an organic group which has 1-12 carbon atoms and symbol A represents a group constituted of H, C, N, O, F, S, Si, and / or P], in an amount of 10 ppm or more of the whole nonaqueous electrolyte, andthe positive electrode is any positive electrode selected from the group consisting of the following positive electrode 1 to 5:

1. a positive electrode containing a positive-electrode active material containing manganese;2. a positive electrode containing a positive-electrode active material having a composition represented by the following composition:LixNi(1-γ-z)CoyMzO2  formula (4)wherein M represents at least one element selected from the group consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying 0<x≤1.2; y represents a number satisfying 0.05≤y≤0.5; and z represents a number satisfying 0.01≤z≤0.5;3. a positive electrode containing a positive-electrode active material selected from the group consisting of the following (a) to (d):(a) a positive-electrode active material having a BET specific surface area of from 0.4 m2 / g to 2 m2 / g,(b) a positive-electrode active material having an average primary-particle diameter of from 0.1 μm to 2 μm,(c) a positive-electrode active material having a median diameter d5, of from 1 μm to 2θμm,(d) a positive-electrode active material having a tap density of from 1.3 g / cm3 to 2.7 g / cm3;4. a positive electrode satisfying at least one requirement selected from the group consisting of the following (e) to (f):(e) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material, a conductive material, and a binder on a current collector, wherein the content of the conductive material in the positive-electrode active-material layer is in the range of from 6% by mass to 20% by mass,(f) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the positive-electrode active-material layer has a density in the range of from 1.7 g / cm, to 3.5 g / cm3,(g) is a positive electrode produced by forming a positive-electrode active-material layer comprising a positive-electrode active material and a binder on a current collector, wherein the ratio of the thickness of the positive-electrode active-material layer to the thickness of the current collector, i.e., the value of (thickness of the active-material layer on one side just before impregnation with the nonaqueous electrolyte) / (thickness of the current collector), is in the range of from 1.6 to 20; and5. a positive electrode containing two or more positive-electrode active materials differing in composition.

2. The lithium secondary battery of claim 1, wherein the nonaqueous electrolyte contains the at least one compound selected from the group consisting of cyclic siloxane compounds represented by general formula (1), fluorosilane compounds represented by general formula (2), compounds represented by general formula (3), compounds having an δ-F bond in the molecule, nitric acid salts, nitrous acid salts, monofluorophosphoric acid salts, di fluorophosphoric acid salts, acetic acid salts, and propionic acid salts in an amount of from 0.01% by mass to 5% by mass.

3. The lithium secondary battery of claim 1, wherein the positive-electrode active material contained in the positive electrode 2 comprises a core positive-electrode active material and, adherent to the surface thereof, a substance having a composition different from that of the core positive-electrode active material.

4. The lithium secondary battery of claim 1 wherein the positive-electrode active material contained in the positive electrode 2 has a tap density of from 1.3 g / cm3 to 2.7 g / cm3.

5. The lithium secondary battery of claim 1 wherein the positive-electrode active material contained in the positive electrode 2 has a median diameter d50 of from 1 μm to 20 μm.

6. The lithium secondary battery of claim 1 wherein the positive-electrode active material contained in the positive electrode 2 is a mixture of two or more positive-electrode active materials differing in median diameter.

7. The lithium secondary battery of claim 1 wherein the positive-electrode active material contained in the positive electrode 2 has an average primary-particle diameter of from 0.1 μm to 2 μm.

8. The lithium secondary battery of claim 1 wherein the positive-electrode active material contained in the positive electrode 2 has a BET specific surface area of from 0.4 m2 / g to 2 m2 / g.

9. The lithium secondary battery of claim 1 wherein the positive electrode 2 contains the positive-electrode active material and a positive-electrode active material differing in composition from that positive-electrode active material.

10. The lithium secondary battery of claim 1 wherein the positive electrode 2 comprises a current collector and, formed thereon, a positive-electrode active-material layer comprising the positive-electrode active material and a binder, the content of the positive-electrode active material in the positive-electrode active-material layer being in the range of from 80% by mass to 95% by mass.

11. The lithium secondary battery of claim 1 wherein the positive electrode 2 comprises a current collector and, formed thereon, a positive-electrode active-material layer comprising the positive-electrode active material and a binder, the positive-electrode active-material layer having a density in the range of from 1.5 g / cm3 to 3.5 g / cm.

12. The lithium secondary battery of claim 1 wherein the positive electrode 2 comprises a current collector and, formed thereon, a positive-electrode active-material layer comprising the positive-electrode active material and a binder, the value of (thickness of the positive-electrode active-material layer on one side just before impregnation with the electrolyte) / (thickness of the current collector) being in the range of from 0.5 to 20.

13. The lithium secondary battery of claim 1 which has at least one property selected from the group consisting of the following (1) to (3):(1) the overall electrode area of the positive electrode is at least 2θ times the surface area of the case of the secondary battery;(2) the secondary battery has a direct-current resistance component of 2θ milliohms (mΩ) or less; and(3) the battery elements held in each battery case of the secondary battery have an electric capacity of 3 ampere-hours (Ah) or higher.

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